Need help selecting speedometer gear

[66CCTWIN]

We are finishing work on a bumpside f 100 project , the speedometer is reading way too slow, it's got 3:50 gears in the rear end and a 302 with 3 speed manual transmission with no overdrive. We have a 17 tooth gear on the tranny end of the speedo cable and it all works but reads about 15 mph at 40+ mph Road speed. Does anyone have an idea what gear we will need to make it read closer to actual?? LMC offers gears from 16 to 21 teeth. The rear rims have 15" tires a little fatter and taller than stock, about 29 1/2" tall. Thanks for any help!

[slick4x4]

I've never seen one that far off
I'm wondering if the speedometer itself has some issues
Or something like cable ends slipping

[9Fingers]

Try this link to formulas and a calculator.

Scroll to the top of the page at link below for formulas and calculator.

http://www.tciauto.com/tc/speedometer-g ... ator/#FORD

[oldsmiley]

You may have an automatic transmission shop in your own town that can help. They may have gears on hand to help you go up or down. It won't hurt to check.

[charliemccraney]

I agree that something isn't working right. In order to speed up the speedometer, the gear must have fewer teeth. Since the difference is almost 3x, you would need about a 7 tooth gear on the cable, which does not exist.
Depending on the output shaft gear, your combo should require a 17-21 tooth cable gear.

[bobenhotep]

Something is wrong in the speedometer head or the cable is slipping. I have never seen one that far off.

Dan