alcazar

iTrader: (2)

Travelling every other day from my home in North Lincs to Pinderfields spinal unit, near Wakefield, I use the M180, M18 and the M62.

Out of interest, I've been checking my fuel consumption.

At a steady 70 on the motorway, it averages 52.3 mpg. But at a steady 80, I managed 54.7 mpg.

So i stuck the computer onto instantaneous fuel consumption, and it's the same: doing 70 on the cruise control, the computer hovers around 52-53 mpg on the flat, (which is most of the route).
At 80 on the cruise control, it's around 54-55.

How come? I thought we were told by government that slowing down DEcreased fuel consumption?

dpb

Cant trust computers

alcazar

iTrader: (2)

Ah....but the steady 70/80 journeys were done with full tank to full tank.

dpb

Itll be more stop/start on one the journeys !








Are we having a White Cliffs of Blackpool moment ?

SWRTWannabe

Headwind / tailwind / flatulence wind?

alcazar

iTrader: (2)

So.........no sensible answers then?

I can see this on a single journey on the flat.

Put the cruise control to 70, the onboard computer flickers around 52mpg
Put the cc to 80, it's flickering around 54.5mpg.

And single journeys with it set to either 70, or 80 show the same figures.

BMWhere?

Cars have these funny things called gears. There is an optimum point in each gear for fuel consumption, usually around 2-3000rpm depending on the car. Running either above or below the optimum will use more fuel, hence, sometimes going a bit faster can be more efficient!

mrtheedge2u2

you shouldn't waste your energy..... the guy is a complete tool with all his dumb-*** questions.

alcazar

iTrader: (2)

Ah, mrtheedge2u2, the perennial fukcpig **** of this site.

Even named after a group and a singer...mild homo tendency?

alcazar

iTrader: (2)

Yeah, good answer. Googled it and it said much the same.

BMWhere?

(no google required )

coupe_20vt

iTrader: (2)

As has already been said, you can't really trust car computers. The only way to get the correct mpg figures is to do a brim to brim check.

BMWhere?

That depends on what figure you are considering and how you are measuring distance on a brim to brim test (trusting the on-board distance or measuring externally with GPS or known test-track length).

The on-board distance will generally by somewhat inaccurate and typically over-reading due to speed instrument tolerances (usually set at +10%) as manufacturers must not indicate a slower speed than you are really travelling. If you fit different wheel/tyre sizes and even tyre wear or pressures can lead to differences in the drive train rotations to distance traveled ration. Essentially, a car never really knows the exact distance traveled, therefore fuel consumption can never be truly accurate.

However, the amount of fuel consumed and the wheel rotation rate can be very accurately measured. While the consumption figures will generally show a better fuel consumption than reality, the variation will be consistent. The exact figures are only important if you want to compare two different vehicles, in which case, the only way is to do a test with a controlled amount of fuel and a controlled distance, speed and temperature. However if you want to compare fuel consumption due to different speeds or driving styles, or even air temperatures, switching the radio/air con on or off etc. in the same vehicle, then the on-board computer will give very accurate and consistent data relative to each test.

Remaining range is a different matter though as the on-board computer will have its own algorithm based on remaining fuel level (which could vary with hills etc), recent average consumption, long term average consumption and some safety margin so you don't actually run out of fuel when it still says you have a range of 1 mile!

coupe_20vt

iTrader: (2)

Yes, but if like the OP you're doing the same trip, in the same car, with the same wheels, you're likely to get 2 comparable figures.

The only way you'll ever get a true mpg figure is in a lab.

BMWhere?

which is exactly what I said in the following paragraph to the one you quoted!

alcazar

iTrader: (2)

I did it as accurately as I could: brimmed it, did the trip, re-brimmed it. Checked exactly how much diesel I'd put in and read the mileage off the odometer, which also tallies with Google Maps..

The two days were similar, hardly any wind and warm and dry.