Speedometer is driving me crazy- Something is Wrong

My car has the same size tires.

originally it was a 2.93 rear and had a 31 tooth speedo gear

speedo was within a few mph ( maybe +/-2)

I swapped the rear to a 3.23 which called for a 33 tooth (based on above charts) but the speed was off more than I wanted. So I went to a 34 and it is in that +/- 2 again.

you will never get it 100% spot on, tires wear, the size is a nominal size.

if it's an automatic there is a percentage RPM increase due to converter slippage,

there are so many ways to calculate the correct gear and all are an approximation.

fyi

when I went from 2.93 to 3.23 that is a 9.3% difference

from 31 to 34 tooth is an 8.8% difference

that's as close as I'm going to get.



As others have said your speedo might be off internally.

a shop can spin it at a specific speed which will give you 1 mile per minute, then adjust the needle to be 60 mph.

the adjustment is deep in the head so nothing you can do on the fly.