Wondered how accurate the 'onboard' computer is on measuring M.P.G. in real terms .. i.e. does the onboard computer match with a tank/brim to tank/brim full? .. specifically Greenline 1.2 diesel?

I have a diesel Yeti and a VRS Fabia, I monitor both closely, the Yeti when I calculate brim to brim fill ups is usually about 0.2 to 0.5 out, car says 43.2 and my calculation gets 42.7 as for the Fabia it is spot on, says 38mpg on the trip and I calculate 38 mpg.

Generally they over read by up to 10%.

I have found that the cars that I have owned have been around 5% optimistic on the trip computer.

Like James I the computer on my Fabia TDi 105 PS is surprisingly accurate. My overall fuel consumption brim-to-brim last time was 58.9 mpg while the computer gave 59.1 mpg. I do the calculation using memory 2 over two or three tankfuls so it should be quite accurate.

I have checked mine regularly and find the on board trip to very accurate. But it may also depend on driving style. Heavy footed people or people with differing driving styles sometimes report slightly less accuracy. The onboard trip I think copes less well with heavy and frequent variances in fuel flow caused by going on and off the throttle. There is always some loss of measurement inbetween the 'on' and 'off' bit.

Of the three Skodas I've owned the first (Mk1 Fab estate 1.9TDi) was pretty much accurate and the last two have been around 7% optimistic...

For my 1.6TDi CR 105 the car computed overall mpg is always within 1mpg of my calculated figure.

My previous car was an Audi A4 with the 1.9TDi PD engine. The car computed mpg was always 6-8% higher than my calculations.

On some French cars the trip computer does the calculations in metric then converts them to imperial (you can tell as some of the trip computers have a max reading of 9999 km) so you tend to get rounding errors. Don't know if VAG work things the same way.