The 56mph Fuel Economy Benchmark.

[ringway]

The 56mph Benchmark.

Why is 56mph the magic number in fuel economy testing?

Why not 50 or 60mph?

A petrol engine 1100cc travelling at a steady 56mph would be using say, 75% of its total power output. This frugal car would still average perhaps 40mpg at this speed.

A 3300cc petrol engine travelling at the same speed would be cantering and using say 25% of its total power output. This gas guzzler would still achieve perhaps 37mpg.

Some 1400cc diesels achieve fantastic returns of 65mpg at 56mph whereas a 3300cc modern diesel would still be asleep at that speed and return perhaps 50mpg.

I’m sure the 56mph figure is from times long past when the average cars had an engine size of say 1600cc.


Is there a better way to calculate?

 

[KillerHERTZ]

Regarding the benchmark, 56Mph is 90Kph.

Same reason why we have 0-62 times (100Kph)

Good average speed for calculating the overall economy of a car?

 

[st4]

No, best MPG is returned in the lowest speed (without labouring the engine) in the highest gear. In my car this is about 50mph.

 

[wemorgan]

here's some information on the fuel effeciency tests, where there's no mention of a 56mph test:

VCAcarfueldata.org.uk - The Fuel Consumption Testing Scheme

 

[scumbag]

some additional info here.

Fuel economy in automobiles - Wikipedia, the free encyclopedia

the c63 would have been able to achieve over 30mpg had I had a decent run without idiots and the weather causing a bit of inefficient driving.

but it did seem to hover around the 28.9mpg when being driven between 48 and 63mph.

another thing I noticed. in "M" it drops out of 7th at 54mph and does not seem to want to go back into it until nearer 60, but in "C" it will , when requested, upshift at 46mph and holds it. problem is, with cruise control on, any slight gradient drops it to 6th. selecting "M" keeps it in 7th. so it takes abit of button prodding.

have not calcultated the full 1200mile trip average as yet.

 

[hawk20]

Interesting to note that the average speed for the extra urban cycle is 39 mph (and not 56 and so many think). It is also not a 'steady, or constant, speed' but does involve acclerating and decelerating.

I have found with all the many MB models I have owned that they will all quite easily achieve the 'combined' figure when on a reasonably long run, with plenty of cruising around the 70mph mark. Numerous posts here and on other MB websites confirm that the combined figure is a reasonable expectation 'on a run'.

Hard acceleration, hills and traffic hugely affect mpg figures in my experience. So does going over 70.

With MB diesels it is possible to drive long distances without exceeding much over 2,000 revs (which in top gear gives about 70mph). Keeping the revs down seems to help economy a lot.

 

[corned]

56mph is purely an arbitrary figure to get some kind of level playing field with fuel figures.

But of course, rolling resistance, aerodynamics, gearing, outside air temp/density, engine size, body mass, torque curves, etc will all impact on the true optimum figure for each different car model and engine size.

I remember a What Car test from about 1974 which had, among other models, a Ford Escort 1100 and RS2000 (both Mk1). The RS2000 was considerably more economical at 70mph than the 1100. Not so at 50mph and under though, obviously...

 

[Stratman]

I recall when these 'Government Figures' were introduced. Not long afterwards, the road tests of cars which were aimed at the fleet/company car market started complaining of a flat spot between 50 and 60 mph.


Nothing to do with manafacturers tweaking injection systems to give a good figure for the tests of course.

 

[jimti]

I wonder if that is why trucks are limited to 56 MPH

 

[Gridlock]

And of course the 7- and 8-speed gearboxes, as already alluded to, are designed to pass emissions tests, not reduce emissions.

 

[corned]

Actually, that's a good point.

The emissions tests are done on rolling road (i.e. artificial) conditions rather than real on-the-road conditions.

So I suppose the manufacturers can provide a car which runs the rolling road at 56mph whilst the engine is doing all of 300rpm in top and producing virtually no emissions...

 

[ringway]

some additional info here.

Fuel economy in automobiles - Wikipedia, the free encyclopedia

the c63 would have been able to achieve over 30mpg had I had a decent run without idiots and the weather causing a bit of inefficient driving.

but it did seem to hover around the 28.9mpg when being driven between 48 and 63mph.

Good going for an engine that size and proof of quality engineering.

My 3.0 Omega (although older) doesnt manage much more than that with an engine less than half the size.

56mph is purely an arbitrary figure to get some kind of level playing field with fuel figures.

I remember a What Car test from about 1974 which had, among other models, a Ford Escort 1100 and RS2000 (both Mk1). The RS2000 was considerably more economical at 70mph than the 1100. Not so at 50mph and under though, obviously...

Yep, so the optimum for the RS2000 is 70mph whereas the 1100 would be at near full power, although it would actually be using more fuel at 70mph than the 1100 at 50mph. I think?

 

[corned]

Yep, so the optimum for the RS2000 is 70mph although it would actually be using more fuel at 70mph than the 1100 at 50mph. I think?

Well, not optimum at 70mph - I'm not sure any car would have its optimum at that kind of speed due to wind resistance.

It was just that the RS was more economical than the 1100 at that speed due to the fact that the RS was cruising, but the 1100 was straining. The optimum speed for both models would probably be very similar. If you follow me?

 

[Rory]

No, best MPG is returned in the lowest speed (without labouring the engine) in the highest gear. In my car this is about 50mph.

Is yours a manual?

I wonder how much efficiency is lost in the autos as they don't fully lock-up until quite high speeds?

 

[oldcro]

No, best MPG is returned in the lowest speed (without labouring the engine) in the highest gear. In my car this is about 50mph.

Come on Steve, when did you ever drive at 50mph long enough to check your mpg?

 

[ringway]

Come on Steve, when did you ever drive at 50mph long enough to check your mpg?

When slowing down after a tug from a lady in uniform.

 

[ringway]

Well, not optimum at 70mph - I'm not sure any car would have its optimum at that kind of speed due to wind resistance.

It was just that the RS was more economical than the 1100 at that speed due to the fact that the RS was cruising, but the 1100 was straining. The optimum speed for both models would probably be very similar. If you follow me?

Kind of..

My brain is still trying to come out of the festive haze.

 

[hawk20]

56mph is purely an arbitrary figure to get some kind of level playing field with fuel figures.

...

In fact the claimed use of 56 mph for the extra urban cycle is a complete myth. See my earlier posting. The average speed for this part of the test is actually 39mph and there is considerable variation as well.

 

[stevesey]

In fact the claimed use of 56 mph for the extra urban cycle is a complete myth. See my earlier posting. The average speed for this part of the test is actually 39mph and there is considerable variation as well.

But the "old" goverment figure was a steady 56mph (plus urban and 75 wasn't it?) so history, rather than myth.

 

[hawk20]

But the "old" goverment figure was a steady 56mph (plus urban and 75 wasn't it?) so history, rather than myth.

It is a myth about the current test and I'm, sure you're right that it is people remembering the old test that has triggered this.