I have had my '58 2.2d for a year now (12,000 miles) and for the first time I decided to check the amount of fuel really used against the computer reading.

Actual consumption over 420 miles worked out at 44.2mpg. The computer over the same period reads 52.3mpg. Most of this mileage was done on a trip from Newcastle, Edinburgh, Glasgow and back to Newcastle at fairly high speeds. Wouldn't like to say exactly how high in print but I am not really unhappy with with 44mpg.

However I was really surprised at the discrepancy in the consumption figures. Does anyone know if this is typical? Is this a big enough error to class as a fault?

In my experience of onboard computer (OBC) fuel calculating systems I wouldn't go as far to say they are liars but definintely economic with the truth. In the past I owned a Renault and twiddling with the OBC I could get reading from zero to 79mpg - Crazy I know.!! I have also tried similar sytems on other makes and models with similar results.

What you have to remember the system is only extrapulating from existing engine data such as engine speeds, throttle and fuel settings, miles travelled, etc, - then it tries to average it out over a peroiod of time It doesn't even have access to a fuel flow meter. So in all honesty the most reliable way of calculating is to do a brim to brim test and divide the number of miles travelled by the fuel used. Even this method can have variances so you would need to do it over a number of trials.

See the link below for an online calculator.

[url="http://www.torquecars.com/tools/uk-mpg-calculator.php"]http://www.torquecars.com/tools/uk-mpg-calculator.php[/url]

See the link below for an online calculator.

[url="http://www.torquecars.com/tools/uk-mpg-calculator.php"]http://www.torquecars.com/tools/uk-mpg-calculator.php[/url]
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Interesting thoughts James and a quick and easy calculator.

I took the ricer or racer test on the website and guess what? It recommends I try something like a performance diesel or a turbo petrol. Current car 150bhp diesel. Last car 140bhp Octavia diesel. Previous to that 180bhp Octavia vRS turbo petrol. Uncanny.

Thanks

Mike