Car Speedos over read by 5 or 10 percent. If the odometer does likewise then that accounts for most of the difference.
See, I thought this at first. I actually started typing a reply then deleted it because I realised – if the odometer is throwing out the computer’s calculations, it’ll be throwing out the OP’s manual calculations also.
I went through this thought process before I started the thread.
My thinking is that the odometer will give a consistent figure to both computer and driver calcs so won’t affect the result either way.
The maths to work out MPG is very straightforward and the same for both calculations so on this basis the only figure that can be variable (in this case) is the actual amount of fuel used over a given period/mileage.
As forecourt fuel pumps are calibrated and checked by trading standards/weights and measures people it must be the fuel useage sensor thing in the vehicle that is potentially inaccurate.