I have an 82 pickup with 4.10 gears. It had 28.5 bfg k0 on it and the speedo was accurate. Just switched to 31 bfg k02 and the speedo is at about a 10 Percent difference so if the speedo is at 50, I'm really going 55. Does anyone know what size speedo gear i need to correct this.

 

I bought a different speedo gear when I changed tire sizes. It worked perfect for about 2 days. The new gear was smaller (to spin the cable faster and correct the speedometer) and it didn't fully engage the drive gear in the transfer case output. It ended up stripping and not spinning at all due to partial engagement. I have gone back to the stock gear. Looks like I'm going to have to actually re-gear the differentials.

 

I haven't heard of many people doing this. Maybe the striping gear is the reason. Maybe call around to some of the big name shops. Marlin, LCE, etc. pick their brains.

Mine is off by about the 10% and I just adjust in my head. Once I lift it and put my tires on then I will regear my diffs.