The speedos are matched from the factory to the diff gear. If you deviate from the stock ratio you need to swap the speedo along with the diff. if you want it to be accurate. Same is true for tires. If you change the diameter significantly you'll throw the speedo off. a 3.07 came in the 300 turbo and the 300 SD until 85 which it changed to 2.88.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.