Speedo Accuracy Calculations

[4129R]

I think my speedo is under-reading so I did some calculations.

For a 5 speed box on a TC, the gearing in 5th is 3.289 and 185/70/13 tyres revolve 870 times per mile. So the engine revolves 870 x 3.289 = 2861.43 times per mile in 5th.

Therefore at 1000 rpm, in 5th gear you are doing (1000/2861.43) x 60 miles per hour = 20.97 mph.

In 4th the gearing is 4.574, so you are doing 15.08 mph.

For a 4 speed box the gearing for 4th is 3.666 which equates to 18.81 mph, and in 3rd, the gearing is 5.268 which equates to 13.09 mph.

(This information is as much for my future benefit as anyone else's!)