Hey guys, I have a 1989 F-150 manual 4X4 with the 4.9 that is lifted. I want to fix the speedometer so that it reads correctly with the larger tires. I have looked at the many forums covering this using the formula: drive gear x axle ratio x tire rev/mile ÷ 1000. The truck has 3.08s, I pulled the driven gear for the speedometer in the T-case it was yellow (18 teeth) and the revolutions per mile for my 315/75/16s are 601/mile. When I do the math: Drive gear (8) X 3.08 X 601 ÷ 1000 I get 14.80864. So I guess my question is what driven gear should I get? The lowest I'm seeing is 16 teeth - Wine- C0DZ-17271-A. I apologize if I'm doing something wrong or am an idiot. I'm about to get out of the military and would like a properly working speedometer for the long drive home.

Thanks in advance

 

i figured it at 583 revs per mile. and you need a 14.xx gear. Smallest i have found is 16 that is a cataloged item.

 

3.08's, 35's and a 4.9?
For a speedometer you could just ask the person driving the tow vehicle how fast you're going. That truck must be a dog.

Your best bet is 4.10 gears. Your speedo with be close and your transmission will thank you.

 

it would just feel like a 302 truck with oem tires.

 

That's how you get coffee in a keyboard...

 

If you can't get there with a change in drive gear alone, you could use a different gear in combination with a speedometer ratio adapter.


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