Speedo Pinion

[Blusmbl]

I went through this with my 65 Mustang. Here’s what I did:

I bought an assortment of the gears and smartly played musical gears, until I had the closest reading higher than true. I used gps and mileage markers for truth data for the speedometer and odometer.

I had to do this as well. I swapped the rearend in my '68, went from 4.30's to 4.10's with a 30" tall tire. I bought the correct gear based on the Brewer's chart (36) , and when I put it in the speedometer still wasn't right. I had to go up 2 teeth to get it to read correctly and match the GPS on my phone. Different tire types and different air pressures change the effective radius of the tire, so the gear has to be adjusted accordingly.