The only mistakes I can see is your calculations of 2083mm/min should be 2028m/min, and then 2.028km per minute which is 121.7km/h, so not a huge difference.


But the other thing is unless you have a recalibrated speedo it is going to be under measuring your speed by about 7km/h at those speeds due to your larger diameter tyres (the speedo still thinks you've got standard 14 inch on there). So you're seeing 100km/h on the speedo but really doing 107km/h. Somewhere between inaccurate speedos and inaccurate tachos would explain the other 14km/h.


If you really wanted to be certain on the ratio redo your test but turn the wheel 10 times and count the tailshaft revolutions (will be more definite as to 3.08 v 3.36)