I drive a 56 plate focus 1.6 tdci sport. I went to meadowhall today and got on at Jun 29a driving through the 50mph section and thought i would see what mpg my car would do with the on board computer it went up to 66.7 mpg that can't be right. I thought it would do about 50 to 55ish but 66 can't be right. So how true or not true is this computer.

I imagine it takes averages between the highest and the lowest when working it out for a published mpg, I think you just experienced one of the highs, I doubt it would be able to maintain that mpg for a sustained period of time.

Sent from my iPad using Ford OC

They are never accurate. I use fuelly.com to track my MPG. 45.1mpg average on my Mk3 Mondeo 2.0 TDCi over 23000 miles.

I always find they are about 5-8% out on dash.

Cheers chaps

When i had stoped it was 66 but when i restarted my car later it had gone down to 58, so i recon its doing 50 to gallon give or take a few

If you reset the trip whilst on the motorway or just before, the car is most economical on a longer steady run, so all you are seeing is the better end of the average MPG first, then when you leave it longer and start driving in town etc you'll see it drop down to the truer average.