speedo gear question

[Phil76]

I'm getting very close to getting the Bronco on the road, and have a quick question about making sure I have the correct speedo gear in place. I am running 33 x 11.50 x15's with a C4/D20, 2.5" lift, and 4:11 gears. What color/teeth do I need to have for this setup? Thanks.

 

[Viperwolf1]

You have no absolute way of knowing until you compare the indicated speed (speedo) with actual speed (GPS) and see what driven gear you have now.

 

[Phil76]

I figured you'd have some wisdom for me. I'll do that. I had an issue recently the last time I ran the Bronco down to the mailbox. Doing what should have been 40 mph, or so, the speedo was only reading 10 mph. It goes back to 0 when stopped, so it is moving. Any thoughts as to what could cause this...something in the cable, or TC?

 

[Viperwolf1]

Could be a few things. Loose rear output assembly in tcase, damaged cable, problem in speedo head.

 

[Quick & Dirty]

Sounds like a bigger issue than speedo tooth count. However;

You can do a good estimate of what driven gear you need if everything else is in good order.
Speedometers are calibrated to 1000 rpm = 60 mph
Look up the revs/mile for the tires you have. The only 33x11.5 tire I found was the Thornbird which is 638 revs/mile. Multiply by gear ratio.

638 x 4.11 = 2622 rpm at 60mph (1 mile per minute)

So you need a gear ratio of 2.62 to get 1000 rpm at the speedo.
7 tooth is the most common drive gear, so 2.62 x 7 = 18.35
An 18 tooth driven gear will get you close.