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Gas Mileage Gears

Gas Mileage Gears

Gas Mileage Gears

(OP)
I am not sure if I should post this in the gears section or the engine/fuel section, but I will start here.

the question is around gears and gas mileage.  The discussion came up during a conversation about corvettes (which I happen to road race).

Corvettes from the factory come with 3.42 gears and 6 speeds manual tranmissions.  Many corvette owners opt for 4.10 gears over the 3.42 gears because it lowers 1/4 mile times between 3-4 tenths.  Everyone agrees with that which is great. The place that many disgree is fuel economy though. Many people say that fuel economy is not affected with 4.10 gears because of engine efficiency do to load on the engine while others state that fuel economy has to be affected since at 60 mph 3.42 gears are 400 less rpms than 4.10 gears.  

Things I know

4.10 gears dyno 10 rwhp less than 3.42 gears
4.10 gears rpms are higher than 3.42 at the same speed in the same gear.

My thought is if the car dynos lower it then requires more energy to get it rolling from a stop.  I also think that if a car is turning higher rpms with the new gearset then more energy is being used.  More energy for starts and more energy for crusing in my little pea mind means less fuel economy.

Car tuners though are saying that with the 4.10 gears they see signs that the car has less load and thus they can lean the car out more.  Because of that reasoning the car is running more efficient with the new gears and thus the fuel economy is not less, but the same.

so I decided to ask the people that really should know which are you guys.

RE: Gas Mileage Gears

I wouldn't answer your efficiency question.  It depends on too many factors.  Speed you drive, BSFC vs. speed and load of the engine, when you shift transmission gears, etc.  However, the 10 less rwhp is almost certainly an effect of bad dyno operation (miscalculation by the operator) and not a real world horsepower loss.     

RE: Gas Mileage Gears

(OP)
Sorry RWHP - stands for Rear Wheel Horsepower

We dyno tons of corvettes and all of them result in about 10 rear wheel horsepower loss when comparing 3.42 gears to 4.10 gears.

RE: Gas Mileage Gears

Keep it simple.  The reason for the vette having 6 gears is to get an overall ratio that lowers the rpms at high speed in order to reduce fuel consumption.  You could have a 1 to 1 rear end, if the gears in the transmision were adjust by the 4.1 or 3.42. Think about FWD cars, they have no rear end gears.

So you are correct in your assumption on cars running at higher RPMs means more fuel, more energy, more acceleration.

What I can not see is that changing the gears affecting RWH.  HP at the engine is torque times RPM, at the wheels, it's the loss of torque being consummed to accelerate the flywheel, shaft, and tires. I don't know how the WHP system works, that would be fun to learn though, does it use pumps and brakes? If not, then I agree with R1100s.



RE: Gas Mileage Gears

My guess is that WW is using a chassis dyno, and is comparing two plots of horsepower (derived from the angular acceleration of a weighted drum which the drive wheels rub against) versus engine rpm measured by counting ignition pulses.  Some of the horsepower difference might be due to "scaling" of the x-axis of one curve relative to the other, as a result of gear ratio differences.  The same mistake was published in a hotrod magazine one time when they compared power output w/two different tire diameters.

My guess is that using the lower-rpm setup will result in slightly better fuel economy at cruise.  The effect might not be large enough to be readily observable via single-tank fuel economy comparisons.  Also, I wonder whether the speedometer and odometer are being corrected for the new gear ratio (presumably changed @ rear diff), or if people are cruising down the highway at lower speeds because the 4.10 gears cause the speedo to read high?

RE: Gas Mileage Gears

Are the 4:10 gears factory installed or aftermarket.  It may be that GM reprograms the ECM for less peak torque, and less peak power, with the lower gears installed.  However, if you take a car with 3.42 gears, pull them out, and install 4.10 gears, you will not effect rwhp.  Rear wheel torque, yes, but rwhp, no.  

RE: Gas Mileage Gears

(OP)
corvette speed is registered from the wheel speed sensor so changing the gears has no affect on the speedo.  chaning the tire size would have an effect.

An Automatic though would require a change in shift points to prevent over-revving.

The dyno is an above ground chassis dyno and works as stated.  It is in the process of being switched out to an inground load bearing dyno which allows our tuner to do the full tune by himself (all cars are tuned for the street and also tuned on a dyno for Wide Open Throttle - WOT).  

RE: Gas Mileage Gears

Do you get similar differences in measured rwhp in different gears?

RE: Gas Mileage Gears

(OP)
4.10s are installed aftermarket not OEM.  Options for a 6 speed mn6 are only 3.42s (a4 have 2.73 or upgrade performance to 3.15).

Isnt horespower a function of torque

hp = tq * rpm / 5250

if so then if tq is lower than so must hp.

RE: Gas Mileage Gears

(OP)
the 10 rwhp would be 3.42 to 4.10s.  if you changed from 3.42 to 3.73 it would be less loss.

Maybe it is just the result of the way the dyno works and calculates both.

RE: Gas Mileage Gears

Chassis dynos do not measure horsepower.  Period.  They measure rate of acceleration on a known inertia then throw in some fudge factors.  You will always get bigger HP numbers with taller gears (assuming the drive train efficiency is constant) because there is a lower rate of acceleration and thus less HP "loss" in accelerating the drive train.

Taller gearing will normally give better efficiency because the engine will be running reduced revs at a larger throttle opening thus less frictional & pumping losses.  However, in the case of the Vette with it's overdrive in both 5th & 6th gears the difference may be very small or negligible.  In any case the Vette gets great highway mileage due to it's low weight, good aerodynamics, tall gearing and highly developed motor.  It is the one GM vehicle I actually admire.

RE: Gas Mileage Gears

(OP)
yes it doesnt measure hp I just talked to our tuner about this (this is why I am inside the car and not under the hood).  The calculation of hp is rate of climb of rpm versus mph increase or something like that.

Based on talking to him I think I understand.  

Fundamentally speaking a car that is running higher rpms with the same load will use more gas the caveat here is yes the car is turning more rpms, but it is now in a lighter load cell (do to the higher gear ratio) in the PCM mapping of load vs rpm thus the car requires less fuel.

He then went on to say some other jibberish that I could not understand - lol.  Something about rpms outweighing load cell values and that affecting gas mileage.

I guess with modern cars you can play with so many variables that you can make cars very efficient compared to the old days of adjusting carbs.

RE: Gas Mileage Gears

(OP)
BTW - thanks to everyone that responded.  On other boards people attack you here I got answers without attacks and that makes it much easier to understand.

RE: Gas Mileage Gears

I have a GMC truck that was just switched from 3.42 to 4.10 gears for improved towing performance. Even though the engine is turning 2500 RPM at speed on the highway instead of the 2000 RPM with the 3.42 gears, the gas mileage remained constant. In town mileage appears to be a little better. I can only attribute the change to the engine running in a more efficient portion of the power band.

RE: Gas Mileage Gears

Personal attacks and non constructive criticism are against the rules here and those who indulge in flame wars soon dissapere.

That is why those of us interested in discussing ideas and learning keep coming back.

I agree with what seems to be the consensus.

Here is my summation, repeating or expanding on some ideas already stated.

Same constant speed equals same load.

Very generally lower rpm equals lower fuel consumption due to lower pumping losses and less friction and less losses from inertia in reciprocating parts in the engine.

There are to many variables to make hard and fast rules as thermal efficiency and friction in the engine changes with manifold pressure.

Different gear sets do have different friction losses, but 10 hp sounds like a lot. These are to do with sliding action of gear teeth on each other and different torque and speed on gears on input side for same speed and torque output to the axle.

I don't know the individual gear ratios in a Corvette, but if the ratios are typical to those offered by GM in their gen 111 V8 powered cars here (Australia) then they are about right for highway cruise in 5th and overgeared in 6th and have no effective low gear (a critical point to me as I tow speed boats and need to go very slow up a steep ramp with extra load).

With the overgeared 6th gear, I expect a the shorter diff ratio makes little difference if all the upshift points are at the same rpm rather than the same speed as were done with the taller gears.

Changing final drive ratio can give interesting but wrong data unless both the speedometer and the odometer are corrected. Without correcting the odometer, with shorter gears, the recorded mileage increases by a greater margin that the fuel efficiency decreases, thus giving wrong but apparent increase in fuel efficiency. I once used this to effect when a company accountant was giving me a hard time about my running costs vs fleet average. Changed diff from 3.08 to 3.7 something. Suddenly my running costs were slightly higher in real terms but below average according to his data that simply divided costs by odometer reading. Simple solution. Everybody happy, unless he reads this.

Oh. front wheel drive cars do have a final drive reduction gear and differential. It is just located in the front rather than the rear axle assembly.




Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Gas Mileage Gears

I believe gas mileage is tied to motive power.  At the same speed, the same car should require the same motive power, regardless of gear ratio.   The gear ratio will affect the delivery of power to the wheel and the efficiency of the drive train.
A Pontiac GTO with a 6 speed manual transmission (Tremec), a VI speed ratio of 0.57:1 and a final drive ratio of 3.46:1 and 17 wheels will have a motive force required of about 535N at 60MPH that converts into a power requirement of about 22.2HP.   Knowing (or assuming) an overall efficiency of the drive-train one could find the needed fuel flow required and this the mileage.   A Vette will probably have similar motive force and power requirements and the change in final drive could not possibly result in a 10HP (about 50% increase) of motive power.

RE: Gas Mileage Gears

Apart from OEM tests for fuel efficiency and emissions, few dyno tests will be done at anything less than WOT.

I expect the Corvette or Pontiac will make over 400 HP and 10 HP will therefore be something like 2.5%

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Gas Mileage Gears

I can think of very good reasons why you might lose 10 hp in aftermarket diff with a higher ratio than the original diff- Efficiency - diffs are rather inefficient. An OEM diff is likely to have better efficiency than an aftermarket one, and a shorter diff's casing has to be stiffer for a given amount of deflection, which controls the misalignmenet of the gears and hence the efficiency.

As to the effect on fuel economy, it is incredibly complex. As a rule of thumb the longer diff would be expected to better, but as an example, when towing a trailer I know of one car that gets better (measured) mpg with a 4.1 rather than a 3.73, in certain circumstances. It depends very much where you are on the fuel map, which is controlled by your operating condition and the transmission strategy/ratios.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Gas Mileage Gears

Why would GM choose a 3.42 ratio for the Corvette if it weren't affecting mileage?

(My guess would be that a relatively light car with low wind resistance driving on a highway with a powerful engine would generate significant pumping losses at higher rpms).

RE: Gas Mileage Gears

(OP)
The 3.42 gears that Chevy uses has always been a debate.  MY Trailblazer SS has 4.10 gears from GM and my road race vette has 3.73s from GM so it is not like they do not have choices.

I think everyone has to remember the vette is designed for highspeed Autobahn type driving (more like road racing than drag).  3.42 seem to be a good gear for that of driving.  The problem though is top speed in a vette is done in 5th gear not 6th.  The car just doesn't have enough power to get to redline in 6th.

As stated I run 3.73 on the track.  I feel the 3.73 give me that little extra snap, but at the same time I can stay in a single gear much longer.  With 4.10s I would have to upshift and with 3.42 I had to downshift too much.

With a 402 in a corvette I can run Full Watkins Glen in 4th and 5th gears which less shifts means less wear and tear and less chance to miss a shift.  I can concentrate more on braking and driving.

RE: Gas Mileage Gears

As you know, the engine has an efficiency curve and a power curve.  The RPM where the peak efficiency occurs you would want to correlate with the highway speed you normally drive at to get the best highway mileage.  This would take some testing on the dyno & comparing to recorded highway speeds with the car.  Then choose the gear ratios that best correlate with peak efficiency at that speed.  
  But, even with the nice slipstream of a corvette, it's still air resistance that eats mileage, and it's a cubic function (meaning going 70mph eats up a ton more of energy than going 50mph.)  If you drive a little slower on the highway and get the gears that best fit this average crusing speed, then you'll see significant energy savings.  For most vehicles, the peak fuel efficiency occurs between 40 and 55 mph.  Owing to shape, the corvette is probably more on the high end - suitable for 55 mph roads.  (boxes, like the typical SUV's on the roads today, push a lot of air, so they need to go wayy slower to find their peak efficiency speeds).
 My best guess is find the ratio that works best at a slower average highway crusing speed; drive that speed and save the fun for one day a week??
  

RE: Gas Mileage Gears

Thanks Greg!  Still lots of power to push at higher speeds..  

  Maybe if cars were shaped more like airplane fuselages, we'd have better mileage, and less pilot error? winky smile

RE: Gas Mileage Gears

Wind drag force is a function of the square of the velocity.  Wind drag power is a function of the cube of the velocity.   Thus going twice as fast will require 8 times the power.    

RE: Gas Mileage Gears

But you get there in half the time.

RE: Gas Mileage Gears

... thereby saving fuel of couse.  big smile

RE: Gas Mileage Gears

Well, yes, I had assumed that the purpose of transport was to get from A to B, rather than competing to see who could burn a mass of fuel in a given time.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Gas Mileage Gears

Greg

I think you are half right.

The purpose of transport is to get from A to B at a reasonable time and cost. Any trade of between time and cost can usually be best assessed by the individual involved.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Gas Mileage Gears

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

RE: Gas Mileage Gears

"What I can not see is that changing the gears affecting RWH."

I'll bet the higher drivetrain speeds with the shorter gearing result in more drivetrain losses.

RE: Gas Mileage Gears

In my Rx-7 I get the best highway mileage at 60 & 80mph.  This is at approximately 3000rpm and 10inHg of vacuum in 4th and 5th gears respectively.  My instant injector duty stands right around 20% at both speeds.  Because of it's low torque at low engine speeds i have 4.10 gears OEM.  For us it's a switch to 4.33's for the 1/4mile.  In any case, i've found that i get the best mileage at the point where my injector duty & rpm is lowest (duh), and vacuum is highest.  If you have the facilities to install a boost/vacuum (vacuum in your case as i suspect you're naturally aspirated) gauge for the purposes of fuel economy i would recommend it.

RE: Gas Mileage Gears

SpinningDorito, if your RX-7 is a third-gen, you have a relatively low drag coefficient and an engine that hates to loaf/lug. The induction is quite inefficient at lower engine speeds. Anyway, these two effects combine to provide a best mileage at those higher speeds.

I've considered putting taller gears, milder cams, and a stand-alone management system into my Saturn ION. That way, I'll get the benefits of taller gearing (less engine drag) without the problems of taller gearing (engine inefficiency). Since I love electronics and control systems, I'm pretty confident in my ability to put together and tune a Megasquirt, but I can't find gears short of CADing them myself and going to a machine shop. I wonder how long it would take driving the car in order to make up such a cost...

RE: Gas Mileage Gears

It is a third gen, it's easier to get the better mileage at 60 because at 80 just about any hill or acceleration makes me go in to boost.  If you're going to build/tune a megasquirt you should be able to get pretty great mileage at steady state cruise.  Our engines aren't known for fuel efficiency but i know of some second gen n/a guys getting upper 20's to the gallon.  I imagine if you tuned with a wideband and pulled all the fuel you could while maintaining a margin of safety you should make similar or better numbers.

RE: Gas Mileage Gears

Put some thermocouples on the diff. and you'll see where the 10 HP is going.  A 1-1 diff. gear would run much cooler.

GM, and other American manufs., don't appear to understand how  gearing "should" work.  The gaps between gears (as a %) should become less in the higher gears (look at motorcycle gearing).  The Vette has a HUGE gap between 5th and 6th, and a big one between 4th and 5th.

So, you're driving in the mountains and want to pick up a few revs, as you would do with a close-ratio box.  But even with 6 gears you can't do that.  Might as well stick with the awfully geared automatic trans., with it's huge diff. between 3rd and "OD".

Now a Doug Nash/Richmond/whatever 5 speed, THAT's a manual trans!!!

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling
 

RE: Gas Mileage Gears

Kind of off the subject, but way in the beginning dcasto said "Think about FWD cars, they have no rear end gears" Of course they don't have "rear" end gears but unless their design has changed in the last 10years they do have a final drive gear set.

ISZ

RE: Gas Mileage Gears

FWD uses a more efficient planetary gearset instead of a hypoid set which has a lot more tooth sliding.

Gold is for the mistress - silver for the maid
Copper for the craftsman cunning in his trade.
"Good!" said the Baron, sitting in his hall
But iron - cold iron is the master of them all.
Rudyard Kipling
 

RE: Gas Mileage Gears

"GM, and other American manufs., don't appear to understand how  gearing "should" work.  The gaps between gears (as a %) should become less in the higher gears (look at motorcycle gearing).  The Vette has a HUGE gap between 5th and 6th, and a big one between 4th and 5th."

For racing and for pure performance, I agree 100%. Your priority should be in the higher gears where usable horsepower is optimized. You can't put full horsepower to the ground early on; you'll lose traction.

However, I like what GM has done. The first few gears are where the fun is, and the last gears are for the daily commute. I really like this Dr. Jekyll / Mr. Hyde approach. It's the best way to have the same car for the road and track. If you're actually finishing 4th gear and getting ready to shift to 5th on a wide open throttle, redline to redline run, I really hope you're either on an open deserted highway or on a sanctioned race track. With six or seven liters (depending on your model), I think it's stupid to NOT have such a high 6th gear available for mild steady-state cruising. I wish I had such a gear.

RE: Gas Mileage Gears

WickedWeasel wrote: "Corvettes from the factory come with 3.42 gears and 6 speeds manual tranmissions.  Many corvette owners opt for 4.10 gears over the 3.42 gears because it lowers 1/4 mile times between 3-4 tenths.  Everyone agrees with that which is great. The place that many disgree is fuel economy though. Many people say that fuel economy is not affected with 4.10 gears because of engine efficiency do to load on the engine while others state that fuel economy has to be affected since at 60 mph 3.42 gears are 400 less rpms than 4.10 gears."

I think you'll find that most owners switch to the 4.10s because of the T56's very high gearing.  GM (as has been mentioned before) tends to miss the mark on gear ratio mixes.  T56 gear ratios start at 2.66 for first and end with a double OD .50:1.  Contrast that with the 4L60E automatic offering ranges from 3.06 up to .70:1.

I'm currently swapping the 4L60E in my 96 Impala to a T56.  In the process I'm going from the stock 3.08 rear to 4.10s and keeping nearly the SAME overal final drive ratios.

4L60E with 3.08 = 9.42 first and 2.156 OD
T56 with 4.10 = 10.9 first and 2.05 OD

Power shouldn't be affected by gear changes.  Minor discrepancies might be noticed, but 10 hp is pretty optimistic.

MPG is a tough one to pinpoint.  For any combination there is a "best" cruise RPM.  Lowering highway RPMs won't always return better MPG.  Sometimes raising it helps by preventing lugging the engine below the torque peak.

RE: Gas Mileage Gears

M---
As to all FWD using a "more efficient" planetary, etc.,etc.
How about my Austin Mini?  It's certainly a FWD and it uses a simple straight cut crown wheel and pinion with a straight cut four synchro gear set.  I'd be hard pressed to find a more efficient method of power transfer in any other FWD vehicle.

Rod

RE: Gas Mileage Gears

"Chassis dynos do not measure horsepower.  Period.  They measure rate of acceleration on a known inertia then throw in some fudge factors.  You will always get bigger HP numbers with taller gears (assuming the drive train efficiency is constant) because there is a lower rate of acceleration and thus less HP "loss" in accelerating the drive train."

You mean Dynojet Chassis dynos (and other inertia dynos) do not measure HP directly.  Not all chassis dynos use this method.  In fact many modern, quality chassis dynos use very light drums and power absorber of some sort (hydraulic pump or eddy current) with a load cell to actually measure force applied to the rollers.

While I don't have anything useful to add to this discussion on gearing I thought I should clarify this misinformation.

RE: Gas Mileage Gears

Real-world data in an RX-7 showed no difference in fuel economy between 3.91 gearing (factory) and 4.78 gearing.  This was correcting for speedometer error, of course.  Driveshaft harmonics were brought down into highway-speeds range, unfortunately.

I actually did see a slight economy gain with the 4.78's, but this may be because I switched to synthetic lubes in the transmission and differential at the same time.

This was with a carbureted engine.  With my experiences with fuel injection, lower RPM would potentially make for better economy simply because it allows finer control over the fuel injection.

Not having any valvetrain loads certainly couldn't have hurt, either!

 

RE: Gas Mileage Gears

I forgot to add:  The early models had a choice of either a 4 speed or a 5 speed overdrive.  The four speed was the same transmission as the 5 speed, just the 5th gear was replaced with a blanking sleeve.  The EPA fuel economy rating for both transmissions was the same.

Given that top speed was also achieved in 4th gear, I assume that the only reason that they even bothered with an overdrive gear was to placate people who thought that driving 4000rpm to keep up with traffic was a little absurd.

The overdrives got serious when they started using fuel injection...

 

RE: Gas Mileage Gears

I'm not aware of FWD's that use planetary gears for the final drive;  admittedly I haven't had my head into every model.  All the ones I am familiar with use a helical gearset at the differential.  Am I wrong?

Ditto the positive comments on the wide-ratio (vs. close-ratio) Tremec 6-sp's;  any "fun" you could have in 5th and 6th would be at speeds not common even in the wide-open Western US, 4th will take you to 100.  The economy and quiet operation you get in 6th on the interstates is much more valuable.  5th is perfect for loafing along on typical inter-city routes @ 50 - 60 mph.

RE: Gas Mileage Gears

"You mean Dynojet Chassis dynos (and other inertia dynos) do not measure HP directly.  Not all chassis dynos use this method.  In fact many modern, quality chassis dynos use very light drums and power absorber of some sort (hydraulic pump or eddy current) with a load cell to actually measure force applied to the rollers.

While I don't have anything useful to add to this discussion on gearing I thought I should clarify this misinformation. "

Very light drums don't have anything to do with it.  Whether acceleration is being computed from the change in rpm over time against a known inertia or from a net force applied at the tire contact patches still does not account for rotational inertia effects and friction losses within the vehicle.  External force measurement methods can't "see" what's happening upstream.

Rather crudely, [Torque] = [I] * [alpha] means that in the lower gears when you have a larger [alpha] due to the greater mechanical advantage the engine has over the load you necessarily waste more [Torque] accelerating the [I] (which had better be remaining constant).


Norm

RE: Gas Mileage Gears

On a brake type dyno, weight of rollers does not matter as the power is measured at steady staten si inertia in the rotating parts does not apply.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Gas Mileage Gears

My own personal experience with changing axle ratio agrees with DennisP's observations in an earlier post.

Assuming you are driving reasonably conservatively, a higher numerical gear ratio need not seriously reduce fuel economy.
This assumes typical city/suburban driving where vehicle speeds are low and constantly changing. A shorter gear gives more acceptable acceleration with much smaller throttle openings. This would be at relatively low speeds where most of the developed power is used for gentle acceleration to keep up with the general traffic flow.

If the vehicle is geared taller, although engine rpm is reduced, throttle openings will need to be wider and throttle "open" times longer to keep up with traffic flow. It is debatable if total engine airflow is actually less with a much taller gear when you demand a certain rate of acceleration.

Out on the open road, where road speeds are much higher, excessive constantly high engine rpm will definitely hurt economy, but around town this does not appear to be the case.

My advice would be if you drive mostly around town in heavy traffic, go for the 4.10 gears.  If you do a lot of long freeway trips interstate, the 3.42 gears would be my choice.

RE: Gas Mileage Gears

When the OD relay fails on a typical 80s Volvo 240 the mpg can be counted on to drop 2 or 3 or 4 mpg. OD ratio ~ 0.7, with non OD ~ 20 mph/1000 rpm.

Efforts to keep close to torque peak are not really justified these days. Engines with computer FI managment even withoug variable valve timing have torque curves that extend way down there.
http://www.rockcrawler.com/reviews/toyota/4runner_2003/torque_curves.gif

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