sounds like a combination of things you.ve probably got a early xr3i box in with the 4.29 final and possibly got a speedo that is reading right as the old xr3i on standard wheels used to read around 120 at 6000 take away 10% inaccuracy and your back to 108 not that far from what you.ve got given the rev counter could be out 100/200 rpm at that stage your probably back to what your reading. bang a series 2 box in it and you'll notice a big improvement 100mph a touch over 4250 rpm