Fuel Computer V Manual Check Query

[thezoo111]

I have just purchased a 2.2 D4D and been keeping an eye on the fuel economy via the computer against working it out manually by resetting the trip and working out the fuel used over miles travelled and there is a big discrepancy.

The computer tells me that I have achieved an average of 48.4 and the trip indicates 504.3 miles travelled but when I refuelled I put in 54.68 litres (12.04 gallons) and this equates to 41.88 MPG when working it out manually.

Can anybody explain the difference.

Also why does the fuel range increase to reflect how many miles should be left in the tank when the average MPG figure goes up, like it does in other cars I have access to on a regular basis in my job.

Thanks

[mungo]

I have just purchased a 2.2 D4D and been keeping an eye on the fuel economy via the computer against working it out manually by resetting the trip and working out the fuel used over miles travelled and there is a big discrepancy.

The computer tells me that I have achieved an average of 48.4 and the trip indicates 504.3 miles travelled but when I refuelled I put in 54.68 litres (12.04 gallons) and this equates to 41.88 MPG when working it out manually.

Can anybody explain the difference.

Also why does the fuel range increase to reflect how many miles should be left in the tank when the average MPG figure goes up, like it does in other cars I have access to on a regular basis in my job.

Thanks

I assume you are using the same forecourt pump at both ends of the journey? One with a fast flow will go 'bang, it's full' at a different point than another. Can't see it accounting for that discrepancy, mind you.

[thezoo111]

Not used same pump but brimmed the fuel on both occassions.

Is there any chance of there being an issue with overfuelling at the injectors?

[hertsnminds]

The issue is probably with the accuracy of the fuel gauge (or lack of).

See this site http://auto.howstuffworks.com/fuel-gauge.htm/printable

And read these postings discussing other manufacturers fuel computer accuracy

http://www.honestjohn.co.uk/forum/post?t=7749

http://townhall-talk.edmunds.com/direct/view/.ef80652

It could also be that most cars use a base set of metrics for their displays and then convert them as required. For example Citroen start with everything in litres and km then convert to miles and gallons. As Toyota sell many cars in the US they could use miles and US gallons as the base units for their calculations and then convert them to European amounts. Depending upon the rounding errors this could explain some of the differences.

[cabcurtains]

Don't trust them, typically the display shows MPG for the current journey only i.e from when the engine was last started. However not all do this!

My Passat is only 2-3 MPG out over a full tank, it shows 49 and returns 46. The Peugeot i had before would say 58 and returned 50, only the manual calculation can be correct figure. Computers are only there to make you feel happier