fuel economy

forgotmyname

Dipping the clutch and coasting is not the thing to do.

Right gear/speed/revs = best economy.

Slower = better? Maybe but at 80+mph in 6th my MPG improves.

Approx peak torque for my car.

My car does 26mpg, No it does 55mpg. Depends on the journey type and traffic conditions.

Strider590

I got an extra 30 miles out of half a tank this month just by making a concious effort not to "short shift" and by holding 4th gear at 50-60mph instead of knocking it into 5th.

Bare in mind that the fuel gauge is anything but linear, the usual for half a tank (I always brim it) is around 170 miles, today when I parked up it was at 199.8.

I make that an increase of around 5mpg

Daz2009

My 3-litre does anywhere between 18 and 45 depending on how I drive

Ultrasonic

Trebor16 wrote: »

But what car do you have, is it a modern car with fuel injection and an engine management system that would cut the fuel supply when the throttle is lifted?

daveyjp wrote:

Why bother coasting? On a modern fuel injected car Simply lifting off the gas stops fuel flow and useage is then nil. When coasting gas is required to maintain tickover.

Purely from a fuel economy point of view, whether it is better to coast with the clutch in, or in gear with your foot off the accelerator will depend on the situation. The reason being that when in gear there is an engine braking effect. It's easier to think of this for a flat road situation so I'll talk about this first.

On a flat road, if you coast with the clutch in a car will roll a lot further before it stops that if trying the same thing in gear from the same starting speed. So to consider which is better you need to compare using no fuel for a shorter distance versus using very little fuel over a longer distance. It is not obvious which is best. In a diesel car there is more in favour of the coasting approach since the car is heavier (meaning more momentum so that it will roll further before stopping), and the idle fuel consumption is lower than in a petrol.

Rolling down a hill, a higher top speed will be reached with clutch in than in gear with foot off the accelerator. Provided this isn't so fast that you need to brake then the coasting approach may be the most fuel efficient. Here is a video that may be of interest:

http://www.youtube.com/watch?v=CPvv08oFKKY

It is produced by a guy called Vekke who is posts a lot on the www.tdiclub.com forum (see e.g. here).

There are however concerns other than fuel economy to consider: safety and clutch component wear being the most obvious here. I don't have enough knowledge to have firm opinions on either of these.

Ultrasonic

Strider590 wrote: »

I got an extra 30 miles out of half a tank this month just by making a concious effort not to "short shift" and by holding 4th gear at 50-60mph instead of knocking it into 5th.

Bare in mind that the fuel gauge is anything but linear, the usual for half a tank (I always brim it) is around 170 miles, today when I parked up it was at 199.8.

I make that an increase of around 5mpg

Are you saying that you think you've improved fuel economy by how many miles to empty your trip computer is telling you at the half-tank stage, or something else? I can't quite tell from what you've written. If it is the former that really isn't an accurate test. It seems extremely unlikely that your fuel consumption would genuinley be better in 4th gear than 5th above 50 mph.

Edit: also bear in mind that the recent hot weather may also have lead to increased fuel economy (depending on use of A/C).

Strider590

Ultrasonic wrote: »

Are you saying that you think you've improved fuel economy by how many miles to empty your trip computer is telling you at the half-tank stage, or something else? I can't quite tell from what you've written. If it is the former that really isn't an accurate test. It seems extremely unlikely that your fuel consumption would genuinley be better in 4th gear than 5th above 50 mph.

Edit: also bear in mind that the recent hot weather may also have lead to increased fuel economy (depending on use of A/C).

If you'd read the post I made some weeks back about my OBDII reader and the fuel economy being better in lower gears rather than trying to cruise along at 1500 rpm all the time, you'd understand what im talking about. I won't know accurately until I next refuel, but from a brimmed tank, having 199 miles on the clock just as the needle passes through half way IS a notable improvement by any standards..... I've owned the car 4 years, I know the mileage/fuel gauge reading very well indeed.

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.

Ultrasonic

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'd be genuinely interested if you any data to support that.

I'm experimenting with higher revs at the moment but the results from one refill weren't encouraging (but that was mostly motorway driving so not the best test).

One thing I seriously struggle with is that there are very good reasons to expect low revs and high gears to give optimum economy. I've seen no credible explanation for the opposite being better.

goosander

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.

Trebor16

forgotmyname wrote: »

Slower = better? Maybe but at 80+mph in 6th my MPG improves.

Approx peak torque for my car.

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.

cyclonebri1

Coasting although not recommended according to the HC, does have it's place. As Ultrasonic posted, there are times when coasting will use less gas than holding it it gear and letting the engine brake it. Ok Zero fuel used but the free energy gained seems to make up for the tickover revs, especially in high gear going downhill.;)