The speedo is normally designed to read over for UK cars. I have the workshop manual for a MKII UK MR2 and in the procedure for testing the speedo it states that a 70 mph if the speedo actually reads 70 it need to be replaced! Cannot remember the exact figure but I think it needs to read 71-79.
The fuel computer should be more accurate, I would have thought it wouldn't use GPS but the wheel sensors for the speed calculations. However even tyre wear will cause difference, at 100mph the difference between 8mm and 2mm of tread will cause 2-3 mph difference.