How accurate is the on-board computer?

Has anybody done a accurate fuel comparison using litres used (fill up to fill up) and mileage driven using the maths, against the on-board MPG dash computer?

It is not that accurate. The best way to find out is to do fill to fill math. The onboard computer get's accurate after about 5-600 miles maybe, or more. The instant meter may be allright, but that one has no real importance :)

They vary from car to car mines abut 3% over, the only real way to check your fuel consuption is to do a brim to brim check several times and then do the average of those checks

They vary from car to car mines abut 3% over, the only real way to check your fuel consuption is to do a brim to brim check several times and then do the average of those checks

Hi Pete,

If you are correct, at 3% over it is more accurate than I would have expected, at a true 40 mpg the computer would read 41.2 mpg, that’s only 1.2 mpg out.

The computer in my Fiesta is also only about 2-3% different from my manual calculations. That's good enough IMO.

Most modern cars are pretty good on the average now as the injectors are computer controlled and so precise that it's fairly straight forward to measure the fuel coming through accurately. From there it's simple aritmethic against the odometer.

They vary from car to car mines abut 3% over, the only real way to check your fuel consuption is to do a brim to brim check several times and then do the average of those checks

Hi Pete,

If you are correct, at 3% over it is more accurate than I would have expected, at a true 40 mpg the computer would read 41.2 mpg, that’s only 1.2 mpg out.

Correct, but as I say not all cars are the same some can be 10% or more out, my mates 07 mondeo under reads by 10%

Neither the maths method of calculate the MPG or the on-board computer are going to be 100% accurate; there are several uncontrollable measurements that come into play.

1/ Temperature of the fuel at each fill up will effect how much is registered on the garage petrol pump.

2/ Different garage pumps turn off at different points, you would need to use the same pump.

3/ The distance travelled is measured by the same on-board computer as the dashboard MPG readout.

It’s a mine field; but if I worried too much I would never get out of bed, so I will settle for a reasonable estimation of MPG and put the calculator away.

Time to pour a cold beer.

Today I brimmed my tank after 460 miles. Trip computer said 50.7 mpg, the true calculated figure was 51.1

So that's a variance of 0.4 mpg or less than 1% (actually 0.8%)

I can live with that :-)

This weekend I had the opportunity to check the MPG display accuracy against the maths.

240 miles on A & B roads over 4 days (1.8 Petrol)

On-board computer = 40 MPG

Maths with (fill to fill) = 41.5MPG

Pretty close considering all the variables.

This time I mean it. The calculator goes away, the car computer is close enough for me.