Why 56.6mph?

[scrimshaw]

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

 

[GregLocock]

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.

 

[NormPeterson]

Average speed of some driving cycle?


Norm

 

[GregLocock]

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.

 

[evelrod]

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

 

[patprimmer]

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

Regards

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[pauljohn]

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.

 

[patprimmer]

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

Regards

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[IRstuff]

see:

http://www.hybridcars.com/epa-numbers/epas-outdated-process.html

http://www.edmunds.com/advice/specialreports/articles/118986/article.html

TTFN

 

[RossABQ]

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???

 

[GregLocock]

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.

 

[evelrod]

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

 

[GregLocock]

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.

 

[Knap]

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.

 

[scrimshaw]

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

Scrim

 

[RossABQ]

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.

 

[kenvlach]

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.

 

[patprimmer]

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

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[evelrod]

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)! (

Rod

 

[Knap]

Rod

Was the Lotus Cortina sold in US?