How accurate are Onboard mileage computers

[tamrsoft]

This is the first time I have had such a gadget as a feature on my car and I would be interested to know how they work and how accurate they are.
I guess the car can record the mileage covered accurately and with fuel injection technology they should be able to count (or compute) the number of injection cycles over a given distance. Is this how they work ? My car was indicating 49.6 mpg at a constant 70 mph and between 36-39 mpg for urban driving. This all sounds too good to me, has anyone comparable results for a similar model

 

[esox]

My experience is the computer`s are approx 1 - 2 mpg optimistic after comparing to a brim to brim check.
Thats accurate enough...

Dont reset the computer after a cold start or just as you get on a motorway though,that just gives you a false indication - cold starts/short journeys are dire for fuel economy....