fuel economy

Ultrasonic

goosander wrote: »

One reason that you may get better fuel economy at higher revs/low load is volumetric efficiency. VE varies with engine speed depending on the engine design e.g cam profile etc, so different engine designs will achieve peak VE at different speeds.

Thanks. Would I be right to think that this effect would show up on a BSFC plot? What I perhaps should have said above is that while I accept that there will be some situations where lower gears and higher revs will be optimal, such situations are unusual, as indicated by the BSFC plots for most engines. (It's not easy to find these, but the best set I've seen are here if anyone is curious.)

forgotmyname

Trebor16 wrote: »

What are you comparing it with to say it improves at 80mph? At that speed you will have drag to consider which will have a considerable effect on mpg.

Comparing it to my own tests in 5th and 6th gears, With the ODBII reader and the not very accurate trip computer also. (3 - 5mpg out, over a tankful). But my OBDII reader is better and backed up by my excel spreadsheet at every fillup.

6th gear does give better fuel consumption at 80+ for my car. I presume the balance of revs/torque produced = more than enough to overcome the drag for 80mph.

At 85 or 90mph it may start dropping, But i like my licence and will leave those tests for others.

Enjoy your tests

Trebor16

forgotmyname wrote: »

Comparing it to my own tests in 5th and 6th gears, With the ODBII reader and the not very accurate trip computer also. (3 - 5mpg out, over a tankful). But my OBDII reader is better and backed up by my excel spreadsheet at every fillup.

6th gear does give better fuel consumption at 80+ for my car. I presume the balance of revs/torque produced = more than enough to overcome the drag for 80mph.

At 85 or 90mph it may start dropping, But i like my licence and will leave those tests for others.

Enjoy your tests

Have you actually tested the car at 70mph and 80mph in sixth gear to come to that conclusion? According to the Department of Transport a car uses 10% more fuel to travel at 70mph than it does at 60mph. A car will also use 25% more fuel to travel at 85mph than it would at 70mph.

AlexisV

The Department of Transport also say it takes 23 metres to brake at 30mph.

Are their fuel figures based on a Morris Marina from the 70's as well?

cyclonebri1

AlexisV wrote: »

The Department of Transport also say it takes 23 metres to brake at 30mph.

Are their fuel figures based on a Morris Marina from the 70's as well?

No they are not in answer to your question.

The fuel consumption figures are much more recent, plus the laws of aerodynamics have not changed.

jaydeeuk1

I remember reading once that pressing the brake pedal is like burning money, and the worst thing to affect MPG.

I therefore hardly ever brake and after 3 years/ 25k my brake pads have only worn 10%.

Scared plenty of woman/children/wife though.

cyclonebri1

Came from Jeremy Clarkson, quote when trying to break some fuel consumption contest in an audi;

"Never, never ever brake".

The strange think is for a 1 and only time, he was right.:T:T:T

forgotmyname

Trebor16 wrote: »

Have you actually tested the car at 70mph and 80mph in sixth gear to come to that conclusion? According to the Department of Transport a car uses 10% more fuel to travel at 70mph than it does at 60mph. A car will also use 25% more fuel to travel at 85mph than it would at 70mph.

The department of transport are probably the same people that think a loaf of bread costs £20.
And modern cars have 4 gears now and you should get into 4th ASAP.

6th gear is too high on some cars, I presume this is why the figures dont match what they have been telling us since the 70's.

My figures are not just random numbers plucked from this air. They are repeatable many times over.

A38, M6, M42 or the M69.

Ultrasonic

Trebor16 wrote: »

According to the Department of Transport a car uses 10% more fuel to travel at 70mph than it does at 60mph. A car will also use 25% more fuel to travel at 85mph than it would at 70mph.

Those figures probably assume a car with five forward gears, not six, which may be relevant? It would be interesting to know quite what they are based on but they must be some sort of approximation/average. As speed increases aerodynamic drag becomes increasingly dominent, and so I'd guess the relative fuel consumptions at different speeds are likely to vary with how aerodynamic the car is.

Having said all that, like you Trebor16 I still stuggle to believe forgotmyname's claims, but if he really does have brim to brim fuel economy data to confirm them, then I have to concede they are true (for his car). It's hard to say anything more without seeing the data. (Forgotmyname - would you be willing to share this? It would also be interesting to know what car you have.)

I do still firmly believe that it is very atypical for a mass-market road car to have better fuel economy at 80 mph than say 70 mph though (let alone 60 mph). Even for 6 speed cars.

hareng

Strider590 wrote: »

I fell into the trap of "short shifting" to keep the revs down and save fuel, but it turns out to be a falsehood. keeping between 2500rpm and 4000rpm seems to return the best mpg figures.

I too have noticed what i could describe as remarkable fuel consumption by doing exactly that. I found it out by knocking down two gears and holding speed at that in 4th, turbo ready to go, mean while watching the instant economy! To be conclusive i would have to do more long term testing.
I firmly believe it could be the mapping of the ECU and the weight, gearing of the car, mines standard and i do own VAS with plug in OBD readers.

Personally i would never ever buy another car without a trip computer, i drive on it 90% of the time. Soon ease off when see 3mpg.