Author Topic: mpg figures from the trip computer (Read 4713 times)

johnroberts · « **on:** January 12, 2007, 09:12:22 PM »

I always check my fuel consumption between filling the tank to the top and calculating differance from last time I filled the car up. However, the computers figures do not match with those I calculated. Which is more correct, the trip computer or calculated? (I use a spreadsheet in Excel, but tapping away on the calcualtor brings up similar results!). Sometime the difference is as much as 4 or 5mpg!

guest74 · « **Reply #1 on:** January 13, 2007, 08:02:11 PM »

hi there

how do you work it out? for a quick check, i just multiply 0.22 X Litres added, to give me the amount in gallons, then divide this figure by the mileage. not sure if this is correct or not. not sure where i got the 0.22 figure from either! Cheesy doesn’t really matter as my car only does 19 miles to the gallon! thank god for LPG!

re the trip computer. does it average the mpg over a set mileage? my scorpio averages the mpg over 500miles, so sometimes can appear different when worked out use the above method.

hth

Comical Engineer · « **Reply #2 on:** January 13, 2007, 08:44:33 PM »

It’s not unknown for the trip computer to be 5-10% out. My previous Saab usually under-read by 5% or more, the Golf I had before that was pretty accurate. I was told that the Saab’s computer could be recalibrated but never bothered. The FRV seems reasonably accurate but it’s too soon to tell properly yet.

If it’s a long way out I’m sure that your local dealer could recalibrate it for you.

Note that it could also be your odometer out (unlikely unless you have modified tyre sizes but not totally unknown). Petrol pumps are tested periodically by HM Revenue & Customs and Trading Standards and must, by law be accurate to within 0.5% so you may take the fuel figure as accurate.

Hope this helps.

guest102 · « **Reply #3 on:** January 14, 2008, 11:33:13 AM »

As above you can count on the fact that the amount of fuel you get from the pump is fairly accurate - pumps are set to deliver a fraction more than that indicated but too much would damage the profits - so the amount is usually very tightly controlled.
Trip computers will never be accurate - to do so you would have to take into account all the following factors - plus more that I probably haven’t thought of …
Vapourisation of the fuel after it is put in your tank. (fuel storage companies usually allow 1%)
The odometer drive is accurate as it is usually taken from the final drive but does not allow for:
Tyre size - Tyre wear - Tyre ’slip’ in contact with the road surface - tyre pressures, which vary due to temperatures during each trip …..
You could use a GPS system but you would still have to allow for time in standing traffic etc….. give up yet?

JazzyB · « **Reply #4 on:** February 19, 2008, 11:43:11 AM »

i would use the manual method of working out mpg figures as i find the fuel computer to be all over the place!!

culzean · « **Reply #5 on:** October 02, 2008, 11:05:23 AM »

The trip computer on the jazz has an update time of about 10 seconds, that means that every 10 seconds it looks at the present rate of fuel flow and averages it out over the amount of miles on the trip readout.

This means that if you zero the trip readout the mpg figure will respond rapidly to any change in your driving style because it is averaging over a very small mileage, but as the trip reading accumulates more miles the mpg reading will respond more slowly to the fluctuations in driving style/traffic condition.

I normally zero my trip every time I fill up the tank and experiment with different driving styles and gearchange points to see what I can get the reading up to, the best I got was 72mpg.