I've often wondered what causes the difference between computer mpg and actual. I'm fairly consistent at the computer being 10% out (it exaggerates my mpg by 10%). This was pretty much the same on previous cars I've had including a VW Golf.
But here we have a 15% and there are drivers who record a very small percentage difference. Does driving style have anything to do with it?
I think driving style does play a part, the fuel used data is gathered from ECU 'injector open times' and the miles traveled is gathered from speedometer. I know the average MPG trip readout updates about every 10 seconds with a snapshot of the MPG at that time ( you can see this if you zero a trip and watch how often a new figure appears on the readout, as the miles on trip increase it gets more and more sluggish to alter the MPG reading no matter how you drive, must be that any change in instantaneous MPG is averaged over greater mileage so makes less and less difference as trip miles mount up). If you are a particularly aggressive driver it makes sense that things can happen in between those 10 second updates that don't get recorded, but if you are a smooth careful driver the 'snapshots' and the time between snapshots would be more likely to be pretty much the same.
Jocko is the most careful driver here so maybe he can comment on readout vs actual mpg he sees.
More likely explanation though is that the speedo normally reads 'fast' by up to 10% (by law it cannot read slow) so this could fool system that you have done more miles than you have actually covered, so amount of fuel used spread over more miles is better MPG, and that is what the readout normally shows 'optimistic'. Makes you think that trip and total mileage recorded by car is also 'optimistic' so that when you have done 110,000 miles you may only have actually done 100,000
I reset one of the trips every time I fill up to get most accurate readout of MPG.