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Gearing for gas milage

Gearing for gas milage

(OP)
If i went from a 3.70 gear ratio and went to a higher gear for gas milage, what kind of power would i need to make to make up for the gearing loss?

Truck:

Stock 2004 2500 quad cab dodge 4x4 w/35in tires. It has 3.70 rear end in it.

Any suggestion on where i can find info or what i need to figure it up myself would be appriciated.

Thanks

Paul

RE: Gearing for gas milage

if you want gas mileage, put smaller tires w/smoother tread on and forget monkeying with the transmission and/or differential.
 

RE: Gearing for gas milage

(OP)
The reason i ask, is i read an article about a truck that was getting 30+ MPG, 40 if he stayed out of it. Right now for a truck i think i am getting great milage, 20+, but i would like to get better if possible.

RE: Gearing for gas milage

What engine, what transmission. The fact that you are even mentioning 20+ mpg US suggests that it must be the Cummins diesel, because no gasoline engine available in that truck would even remotely touch that no matter what you tried (except downhill with following wind).

My calcs say you are getting 2132 driveshafts revs per mile (= 2132 rpm at 60 mph). I don't know what the overdrive ratio is in your transmission, but if it is in the 0.7 range (typical) then you are turning around 1500 rpm at 60 mph. I don't think going to a shorter (numerically higher) axle ratio is what you want to do. A taller (numerically lower) axle ratio might help a little, but not in proportion to the amount you change the ratio.

If you are just talking about pulling the truck itself, I don't think you need to worry about power, the truck already has far more than it needs. If that truck will pull a 10,000 lb box trailer with the stock ratio then it will have enough to pull a little taller axle ratio without the trailer. If you are using the truck as a truck to pull a big trailer then it might be an issue.

If you put a taller axle ratio then you might have to downshift more on hills, that's all.

RE: Gearing for gas milage

don't believe everything you read...   

RE: Gearing for gas milage

(OP)
Brian

Thanks for the info, that is very helpfull. Yes it is a cummins and it has a 6speed. At 60mph i am running about 1750rpm. The real problem i am having is when i get on the highway and drive 70 or 75mph, i am up around 2000 2100rpm. That is were my as milage starts to drop. If it is possible, i would like better gas milage and still be able to tow what a stock truck can tow.

Correct me if im wrong, but with some tuning, added power, and taller axle gearing, i should be able to increase gas milage.

RE: Gearing for gas milage

tuning maybe, taller gearing maybe, added power definitely not... just drive <65mph when you want to save fuel.


 

RE: Gearing for gas milage

I guess it is already turbocharged so adding a turbo won't help.

Taller gears only helps if it is currently under geared. There is a point of diminishing returns with taller gears. The manufacturers engineers are well aware of this.

The tried and proved methods of reducing fuel consumption are:-

Use Diesel.
Reduce engine size and add a turbo to get the power back.
Reduce power.
Reduce tare weight(no unnecessary accessories or options and light weight body).
Reduce luggage weight (get rid of all unnecessary junk you carry in the truck).
Reduce payload weight (might seriously eat into earnings).
Reduce rotating weight (lighter smaller wheels and tyres).
Reduce rolling resistance (higher tyre pressures and narrower tyres).
Reduce aero drag (get rid of bull bars, unnecessary lights, horns, emblems, roof & gun racks, wide tyres and flares, antennas, mirrors etc).
Reduce frontal area.
Eliminate unecessary trips.
Combine trips to reduce cold starts.
Use light weight synthetic oil.
Run a high temperature thermostat.
Block of portion of radiator grill in cold weather or cold climate region so the opening is no bigger than necessary.
Use a stick shift transmission.
Use lightweight gear and diff oil.
Check brakes are not dragging.
Remove air conditioner.
Minimise draw on electrical system.
I am sure I have forgotten something.

Many of these methods will reduce function, performance or durability of the truck. That is why the OEM may not have done them.

It is amazing that some people can actually believe that the large type American pickup truck can get substantially better mileage than my stick shift, fuel injected 2300lb, low profile, good aerodynamic design, Honda Integra.
 

 

Regards
Pat
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RE: Gearing for gas milage

My experience with automotive-scale modern diesels is with the VW TDI, but concepts will be the same.

Chip-tuning and other MILD tuning of these diesels generally won't hurt the fuel consumption but you need to stay below the black-smoke threshold and you need to watch the EGT (exhaust gas temperature). But, it won't help the consumption either. Under SOME circumstances, it might let you stay in a taller gear and that might help a little. If there are restrictions in the intake or exhaust systems, de-restrict them. Won't hurt, but won't help much, either. Can help a little with controlling EGT. I think the Cummins folks call it BOMBing (Better Off Modified, Baby) ...

Driving faster will always use more fuel because the power to overcome drag needs to come from somewhere no matter what the gearing is. The only possible benefit is to get the engine into a more efficient operating range, but it will only help by a few percent.

I do think you are short-geared the way it is now. What's the next taller available ratio? If you can get it about 10% taller you might gain around 3% in consumption (as you can see, the main benefit is reduced noise and better comfort, as opposed to better consumption).

I've actually done similar to my VW to what you are contemplating ... It's chipped, but I installed a taller 5th-gear ratio (0.681 vs stock 0.744) in the transmission. On the VW 5-speed transmissions, you can change 5th without taking the tranny apart, but the final drive is buried in the transaxle. I like it.

RE: Gearing for gas milage

Ivymike "added power definitely not"

Not necessarily true. Engines today are tuned for best emissions, NOT best efficiency (and thus mileage). In some cases a diesel with mild chip will noticeably increase the efficiency.

I agree with Pat's list. I will add that two other ways to reduce aero drag/frontal area is to lower the truck and add a tonneau cover.

Here is a whitepaper from Cummins. It targets semis, but most if it is applicable.
http://www.everytime.cummins.com/assets/pdf/MPG_Secrets_Whitepaper.pdf

ISZ

RE: Gearing for gas milage

way to snip tidbits for effect...  I think I said "tuning maybe, taller gearing maybe, added power definitely not"

I'd say that improving efficiency by tweaking injection timing counts as "tuning" (even if it's illegal), and dumping gobs of extra fuel in to increase power counts as "added power."  

Are you saying that dumping fuel in until it smokes is the way to improve efficiency?

 

RE: Gearing for gas milage

As this is a ear wheel drive, the final drive is in the differential in the rear axle housing. It has a ratio of 3.7:1 for the crown wheel and pinion gear set.

He is saying he now turns the engine slower to try to obtain better mileage. He now asks how much extra power he needs to add to get back lost performance.

Most replies refute his statement and question the wisdom of his stated intentions.  

Regards
Pat
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RE: Gearing for gas milage

Well, he needs to understand that it is torque, not power he needs to increase. If he runs a higher gear he probably has all the power he needs to accelerate the vehicle because it is overpowered to start with. For small decreases in overall gear ratio he will probably gain when the vehicle is empty because running the engine at a lower rpm will reduce internal friction.

The vehicle itself is not built for milage. Perhaps the best way to get good milage is to simply not drive it unless it is fully loaded and get a Honda Insight for commuting and trips to the store.  

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

RE: Gearing for gas milage

Wouldn't there be a noticable change dropping the rpm's down to around say 1200rpm at 60mph and say 1450rpm at 70mph. The engine should still easily make enough power.

Another change - remove the engine driven cooling fan and install an electric fan.

 

RE: Gearing for gas milage

I knew I missed something. On European and Japanese cars, electric fans only is pretty much across the board in this day and age. Electric water pumps might also give a marginal improvment.
 

Regards
Pat
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RE: Gearing for gas milage

Go for the big stuff.

4WD is a big source of rolling resistance. If possible, free wheeling the front wheels can make a big difference.

Wind resistance is another big contributor.
If you are handy with fiberglass make a top for the bed that tapers down from the cab roof to the top of the tail gate.

Consider fender skirts for the rear wheels.

Put a smooth bottom on the underside, especially under the engine.

Remove the mirrors and use a video rear view mirror. Ditto for any external antennas.

Shutter the radiator so that air flows around the outside instead of through the engine compartment except when cooling is needed. This will also help when it is cold out. This is done on over the road trucks.

Use oils with a flat viscosity curve (synthetics). This may have a big impact on the transfer case and final drives.

Use skinny high pressure tires which will cut down both the rolling resistance and wind resistance of the tires.

Check the alignment and perhaps study the effects of being aligned at the extremes and in the middle of the alignment specs.

With 4wd air pressure can be a big player. Check the effect of different front and rear pressure combinations.

For both the tires and the alignment you can do a preliminary check using a spring scale and a tow rope. See what these changes do to the towing load. Lower is better. An alternative is to find a flat stretch of road and coast from a set speed like 40mph to a stop. Record roll distance in neutral. Do several runs in both directions and average.

Remove any high drain electrical items such as high powered stereos, running lights, etc.

Also remove air conditioning and run with the windows up when on the road.

Operationally consider coasting to a stop in neutral and shut down the engine at lights and when in line at McDs. In stop and go situations don't use the accelerator. Just let the engine idle move you along if safe.

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

RE: Gearing for gas milage

Before you go any further I would see how long it's going to take you to pay back a couple of thousand dollars for swapping gears (unless you can do this yourself properly) in fuel savings.

Try this article for some suggestions but I doubt any of these mods are going to be economical though.  Buying an older small car is likely your best option for saving fuel $.

http://www.fourwheeler.com/techarticles/engine/129_0612_improve_fuel_economy/index.html

Brian Bobyk - Hoerbiger Canada

RE: Gearing for gas milage

Brian

Pay attention

It says earlier that these guys get better mileage out of 2.5 tonne trucks than I can get out of a 1 tonne Honda so wh would they trade down to a gas guzzeling sub compact.  winky smile

Regards
Pat
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RE: Gearing for gas milage

(OP)
Trading is not an option. I use this truck everyday and for what it is, it gets great gas milage. If there is room for improvement and an chance to learn something, then why not. I can do all the work myself. I am just looking for the best place to start.

Thanks guys for all your input!

RE: Gearing for gas milage

Quote:

...At 60mph i am running about 1750rpm. The real problem i am having is when i get on the highway and drive 70 or 75mph,...
Do you think the fact that wind resistance increases 56% from 60 to 75 mph has anything to do with this?

To compensate you have to reduce your drag coefficient by 56%. From the looks of it the best you can hope for is a 30% reduction (see Drag coefficient study #1, Drag coefficient study #2, Drag coefficient study #3).

A big air dam in front and a half height cap seem to be the way to go.

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

RE: Gearing for gas milage

Short stroking a car crusher might reduce the frontal area 56%.

RE: Gearing for gas milage

Maybe I'm way off base here, but I would look for a chart of engine efficiencies versus RPM and gear so that I could keep my RPM in the best part of the curve.
Is this information not available?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Gearing for gas milage

For most vehicles that information is a little hard to come by. You need the mass and rolling resistance, and the drag coefficient and frontal area, and the BSFC map for the engine, and the gear ratios.  

Cheers

Greg Locock

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

RE: Gearing for gas milage

You can put the vehicle on a chassis dyno and obtain that info. If you want to be a purist, put it on a chassis dyno and then remove the engine and put it on a dyno and map the engine both ways. Then compare and the two.  

TOP
CSWP, BSSE
www.engtran.com  www.niswug.org

"Node news is good news."

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