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154 Posts

My 53 Extreme is reading 41.5mpg, this has averaged out over some 2000 miles now and is pretty constant. I used to reset it every week but it changes too quickly and doesn't give a true inpression of average mpg over time, bouncing between 37 and 44mpg all the time, so I've started just resetting my trip computer on fills and nothing else.

The thing is the mpg estimate of 41.5 just doesn't compute. I put £30 worth of diesel in a week, alternating between the BP normal (so I get 33ltrs) or the Ultimate (where I get 30ltrs).

On the 33ltr fill I average 340-360miles, depending how I drive, and with the 30ltr fill I get around 330-340miles, but I've never had less than 330 miles for 30 litres.

Well, at 4.54 litres per gallon this works out at 30l/4.54l = 6.6 Gallons, 330m/6.6g = 50mpg.

or: 33l/4.54l = 7.2 gallons, 360m/7.2g = 50pmg

So the figures are relative, but how the hell can the computer be so far wrong? It's a simple calculation - what's more, it even estimates how much mileage is left in terms of fuel when I fill up correctly (usually within about 10miles either way), so if it knows how far I'm going to get on the fuel I've put in, how can it work out the mpg and be so wrong?

Anyone shed any light on this?