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Why 56.6mph?

Why 56.6mph?

Why 56.6mph?

(OP)

Why is mpg sometimes measured at 56.6mph? I seem to remember this speed was supposed to be the most economical but it can't be the same for all cars so how was it calculated?

thanks

Scrim

RE: Why 56.6mph?

Never heard of that one. And as you say, it is wrong.

Cheers

Greg Locock

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

RE: Why 56.6mph?

Average speed of some driving cycle?  


Norm

RE: Why 56.6mph?

Maybe, but a google search reveals nothing of interest in the first 3 pages.

Cheers

Greg Locock

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

RE: Why 56.6mph?

When I was learning to drive, it was 45 mph.  I believe that, too, was probably wrong for all but perhaps a Model A Ford...I am not too sure about mileage as in the early 50's it just was not important.  Gas retailed at $.025/gal and as low as $.10 in "gas wars".  I paid ~$.19 forever.

Rod

RE: Why 56.6mph?

It's 91 kph, so its not due to conversion from a convenient metric measurement.

Regards

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

RE: Why 56.6mph?

I seem to remember the British Government telling people to drive at this speed in the 70's or 80's as it was the most fuel efficient. How you're supposed to drive at this very specific figure is hard to say!

I did find a few sites on google that talk about fuel economy at 56.6mph (mostly reviews) but all in the UK.

RE: Why 56.6mph?

But it will be very different with different cars, different weather and different altitude.

Regards

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

RE: Why 56.6mph?

Interesting that that first article has a list of Motor Trend's test results for hydrids;  it shows the Honda Insight as getting 50 mpg.  Wasn't it all-electric???

RE: Why 56.6mph?

No. It is a light parallel hybrid.

Cheers

Greg Locock

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

RE: Why 56.6mph?

Interesting comparison, EPA numbers and 'real world' numbers, at least for the Prius.  I was just on the phone with my accountant (he has owned two Prius since their introduction)...He agrees with the '40mpg' number.  His two cars vary between the high 30's to the mid 40's at freeway and local traffic conditions in Anaheim.

Rod

RE: Why 56.6mph?

I think Prius and Impulse were heavily optimised around the old EPA test. For that matter I suspect Marketing paid for the development of both of them!

Back to the original topic, here's a chart of fuel consumption vs speed, sorry about the awful formatting. d means not measured.


Fuel Economy by Speed, 1973, 1984, and 1997 (miles per gallon)

Speed  (miles per hour)
1973 (13 Vehicles)
1984 (15 vehicles)
1997 (8 vehicles)
 
15
 d
 21.1
 22.3
 
20
 d
 25.5
 25.5
 
25
 d
 30.0
 27.5
 
30
 21.1
 31.8
 29.0
 
35
 21.1
 33.6
 28.8
 
40
 21.1
 33.6
 30.0
 
45
 20.3
 33.5
 29.9
 
50
 19.5
 31.9
 30.2
 
55
 18.5
 30.3
 30.4
 
60
 17.5
 27.6
 28.8
 
65
 16.2
 24.9
 27.4
 
70
 14.9
 22.5
 25.3
 
75
 d
 20.0
 25.3

Very interesting that the more modern cars show a distinct improvement at higher speeds. I am told that this is common. As cars get heavier it is increasingly difficult to improve town mileage.

If you ask me we should be driving at 75 by that data.
 


Cheers

Greg Locock

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

RE: Why 56.6mph?

A current recommendation for UK motorways is 78mph, not that it will be changed from 70mph.

During the 1973 fuel crisis when rationing was almost introduced, the UK Government recommended 56mph for most economical motorway speed.

RE: Why 56.6mph?

(OP)

Knap
thanks for that info, I was beginning to think I imagined it. Do you know where the figure came from though?

Scrim

RE: Why 56.6mph?

When the gov't imposed the 55 mph limit in the 70's, I was in school (univ.) and the MechEng dept did some testing that showed semi-trucks got the same mileage at 55 as at 65 or 70, or close enough that it was not worth the reduction, anyway.  It's aerodynamics (lack thereof in this case) and gearing.

RE: Why 56.6mph?

RossABQ, that result for semis only makes sense if they were unable to run in top gear at 55. Their aerodynamics are even more important than for small vehicles, but there is less they can do (other than drafting!).

Interesting list, Greg. Newer vehicles are more streamlined than in 1973, are fuel injected rather than carbureted (I only recall Porsche having FI in 1973), likely have an extra transmission gear, and automatic transmissions 'lock up' in high gear for no torque converter loss. The electronic sensors, platinum-tipped spark plugs, better tires, lower viscosity oils, help too.

But, aerodynamic resistance increases with the velocity squared, so the best fuel economy is at the lowest speed at which the engine can run efficiently in top gear. My old Taurus gave 35 mpg at 55 MPH, with the automatic trans. in overdrive 4th & the engine at 1830 RPM.

RE: Why 56.6mph?

Trucks do have a very small frontal area compared to their fully loaded weight, so acceleration, rolling resistance and climbing hills will be the a relatively larger proportion that aero when compared to cars.

Regards

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

RE: Why 56.6mph?

Here are a couple more mileage numbers for some of my vintage cars. All original engines specs. Just kinda FYI.

1937 Buick Roadmaster (original, ~67K miles)---Los Angeles to San Francisco and back (up on 101 and back on I-5) 10 mpg average (65/70mph).

1930 Model A Ford Std. Coupe (engine rebuilt to OEM specs)---Average 15 to 17 mpg for last 18 years.

1959 Nash Metropolitan (modified with a Datsun 210 carb and a 3.9:1 ring and pinion from an MG Midget...orig. was 4.22)---City, 25+mpg...freeway, 40 mpg max... cruises at 65/70 mph.

1966 Lotus Cortina(stock 1558cc twincam engine)---Long Beach to Amarillo and back on I-40, 21 mpg average.

1960 Austin Healy Sprite (stock 948cc engine)---Long Beach to El Paso and back on I-10, 39.9 mpg average.

Just for 'giggles', 1991 Dodge/Cummins 1 ton duelly, 21/22 mpg for over 200,000 miles and counting. Lowest mileage was towing a 5000 lb trailer to Vegas and that was 18 mpg @ 55 mph. And my 454 BBC powered 27 foot class A @ ~6mpg most of the time (really a killer to the bank account)! :o(

Rod

RE: Why 56.6mph?

Rod

Was the Lotus Cortina sold in US?

RE: Why 56.6mph?

I don't know how true it is but I went to Bonneville Sept 2005 and when I got home I pushed the trip button it read 24.5 MPG @ 93.2MPH It was about 8 hrs each way BTW it is a Z06


Cheers

I don't know anything but the people that do.

RE: Why 56.6mph?

Quote:

I think Prius and Impulse were heavily optimised around the old EPA test. For that matter I suspect Marketing paid for the development of both of them!

Actually their marketing wanted to put more realistic numbers on the window stickers the law won't let them.  They're getting some pissed off customers, but they are forbidden to report anything but the EPA mileage.  The reason the numbers are so high in the EPA test is that it only looks at the fuel burned by measuring emissions from the tailpipe.  It doesn't account for energy coming from the battery.  Obviously any hybrid will get a much better score.

http://www.wired.com/news/autotech/0,2554,63413,00.html

-b

RE: Why 56.6mph?

Kenvlach,  the theory put forward on the semi-trucks was that their aerodynamics were not following the normal parabolic shaped (v-squared) curve;  it was nearly flat between 55-75.  This was pre-KenWorth anteater style tractors (a reference that may be lost on the Europeans here).

RE: Why 56.6mph?

Knap.
The Locor (one of 7 that I have owned) was the U.S. 105 hp version and not the Euro 112 hp. I have owned both. I still have the original, but it was converted to a race car in 1967.  My only rhd car was my 63 Austin Cooper (which was destroyed in 2003) and my current 67 Austin Cooper S which is a full race car only.

Thundair.
Neener, neener, neener.  I made it from Lake Elsinore to Salt Lake City in 7 1/2 hours, without a ticket, I might add (couldn't come close to that on the way back, lost the AC in Vegas and stopped for repair...it was summer).  Car?  Much modded 1983 T coupe. Fuel mileage for the round trip was a tick under 20.

Rod

RE: Why 56.6mph?

I had one of those 'T' coupes 83 and after 4 turbos and 3 motors I gave up but it was a  nice balanced car.

Some one should have told me how to treat a turbo.

I killed one turbo just accidentally hitting it with water at a car wash.

The small engine turbos are the best of both worlds for mileage and power.. I am excited to see some mods to the Ecotec engine with 162 horsepower could be the new small block Chevy...

I guess I am straying from the original post ...I'll stop

I don't know anything but the people that do.

RE: Why 56.6mph?

In the EU cars that are sold are rated against a fueleconomy scale that includes steady state driving at 90 kmh, 120 kmh and an urban cycle. The three measurements are then combined into an overall figure that may decide how much tax you will have to pay when buying that car, based on its fuel consumption and its likely  contribution to air pollution.

Ofcourse the figures are arbitrary but since 90 kmh is the maximum speed on most main roads in Europe and 120 kmh is the maximum speed on motorways in many countries, the mix of the three figures try to convey a real fuel consumption figure to the consumer. In real life the figure you will obtain in practice is usually worse, although the figures are better since all cars are equipped with threeway catalysts that prevent manufacturers running the engines at extremely lean mixtures at the fixed test speeds.

At the moment the tests is no longer run at 90 kmh but only the 120 kmh and the urban cycle have remained

In all EU countries a list is published comparing the various cars available here.

See: http://www.vcacarfueldata.org.uk/index.asp

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