My speedometer has been WAY-OFF after switching from 4.10 to 3.54 gears in my F-250 4X4, I also went from a 29" high tire to a 34" high one, that just added to the problem. By my gps when the speedometer was on 50 my GPS was reading 64 MPH, that equals 28% slow. The speedometer runs off of the tail-shaft on the transfer case and I could not find a internal gear to fix this, I did find a gear ratio adapter that can correct, up or down any amount, you just have to order what you need. I got one to speed up my speedometer 28% for $70 dollars including shipping from (PATC - Speedometer World) and it works perfect, speedometer is now dead on right and I am a Happy Camper. Just passing this along if anyone else has this problem, if this adapter is not the one you need just look at their website, I'm almost sure you can find what you need.

Just for your information I have a C-6 automatic and before changes I was running 2683 RPM at 55 MPH, after switching to lower gears and taller tire it now runs 1976 RPM at 55. and the FE engine cruises just great in the 2000 RPM range.

Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4

 

Thanks for the information. One thing though is that your old 4.10 ratio is a lower gear (higher numerically), compared with your new 3.54 which is a higher gear.

 

Good job TP for building this info thread. Now in the future any Guy or Gal who googles the same speedo problem will be able to see and find your solution. Cool!