I notice when driving my 3 71s, the speedometers read approximately 5 -7 miles faster than actual GPS readings, depending on road speeds. On my 71 SX hardtop and 71 442 I have 225/70-14 tires and on my 71 SX convertible, 235/60-15s, both smaller in diameter than the G70-14s they came with. I understand from reading the searched results the speedometers may have been a bit optimistic contributing to part of the discrepancy noted.
All have 18 tooth drive gears in the TH400s. The convertible has 2:56 gears with a 35 driven gear, the SX hardtop 3:08s with a 43 tooth gear and finally the 442 has 3:23s and a 45 tooth driven gear. Now for my question, approx. how many mph change would there be for each increase in tooth count of the driven gear with the tires I have? I can increase the SXs tooth count, but the 442 already has the highest tooth count before going to an adapter, which I wouldn't bother doing.
I'm thinking of ordering new driven gears but didn't know whether to go up 1 or 2 in tooth count. Any mathematicians out there?

 

Brian, you've done all the hard work already.

It's easy to figure the driven gear from your data. Take the displayed speed, divide by the GPS speed, and multiply by the number of driven teeth; this equals how many teeth you need to correct the mismatch.

For example, if your speedo shows 65 mph, your GPS shows 60 mph, and you have a 35 driven gear... 65/60 x 35 = 38 teeth.

 

 

Thanks for the info, I'll do the calculations.