New tires old speedo gear

[FloridaRanger89]

Hello, I am about to finish my 351 build and want to get new tires soon, I have found the formula to calibrate the speedometer gear when final drive gears are changed. However, I can not find any information on how to determine the amount of teeth I will need based on a larger tire size. Currently I have 31's with 3.73 gears. I would like to get 35-38'' tires depending on location, price, and condition. If anybody could steer me in the right direction that would be great.

 

[BLOODBANE]

On my Ranger, with the 35's the white gear (sorry I dont have the tooth count) put it back almost perfect. I use the Dana 20 case though (speedo is driven off of it). It was the smallest we could find for that application.

 

[FloridaRanger89]

Thanks I believe I have the white gear right now and I am using an NP 435/205 drivetrain but to my understanding the speedo gear makes the difference not the trans/transfer.

 

[RonD]

Do you know how many teeth are on the Drive Gear?
The Drive gear is on the output shaft, a worm gear.

Driven Gear is what you change to match tire size and axle ratio.

This site has calculator if you know the Drive gear tooth count: http://www.tciauto.com/tc/speedometer-gear-calculator/

I think Rangers and explorers had 7 tooth drive gears
NP435/205 probably 6 tooth.

Drive gear used was based on the range of rear axle ratios and stock tire sizes available on a vehicle model that year, so Driven gears available could cover all options
If you know the model and year for the NP435 then you can probably look up the Drive gear tooth count
Drive gears are also color coded like Driven gears so if you can see Drive gear color................
Ford 205 transfer case lists 6 tooth Blue Drive gear, and also 7 tooth White Drive gear