My `86 has a C6 and 3.55 gears like most of them. Does someone know how many teeth the speedo gear has on theirs? Or what color it is? My truck seems to cruise comfortably between 55-65 thru the neighborhood. Thinking the gear may not be correct.

Just took a gps reading to see how far this speedo is off. Got back right as the sky opened up and all hail broke loose.

When speedo reaches 60, the truck is only traveling 50mph.

 

I think mine was orange when I pulled it out.

Ultimately, you'll need to add tire size into your calculation and just buy new. I bought them on summit.

Speedometer Gears - Ford Transmission Make - Free Shipping on Orders Over $99 at Summit Racing

 

Mine is the other way, 46 indicated gives 60gps

 

Try this:
Indicated speed divided by true speed, times the number of teeth on your speedo gear, equals the number of teeth on the speedo gear you want.

 

Wow thats too easy.

I tried it with both gears I have and came with nearly the same answer. With the 16 gear, it needed a 19.2. With 21 gear the correct one is 18.4. So a 19 tooth gear should get me to near perfect mph. THANKS!

I didnt connect the dots before but thats a similar equation I use to get a corrected gear ratio for different size tires.