Speedometer gear

[wolfsden]

What size speedometer gear are you using?
I have a 42 tooth speedometer gear in my TH 400 trans, I'm probably reading 10 miles faster then I'm actually going, also at about 80mph the speedo needle starts to bounce badly. I don't know what size rear I have in the truck and don't want to take it apart to find out. I'm also running 22 inch transit wheels with 265/35 r22 tires that measure 29.3 in height.
I think I need a smaller gear maybe a 38 tooth, any suggestions?

[earl84]

Im sorry i cant remember the business i bought mine from, but they had a calculator on their website to figure this out. It might have asked for your gear ratio, though.

There is a way to jack up your rear wheels and rotate the wheels and compare to driveshaft rotations to figure out the gear ratio, however. This would give you the number to plug into a calculator.

[geezer#99]

Look around here for calculators.
https://transmissioncenter.net/shop/

To figure out what rear gears you have, read here.
https://www.google.ca/webhp?sourceid...d+gear+ratio&*

[wolfsden]

Originally Posted by earl84

Im sorry i cant remember the business i bought mine from, but they had a calculator on their website to figure this out. It might have asked for your gear ratio, though.

There is a way to jack up your rear wheels and rotate the wheels and compare to driveshaft rotations to figure out the gear ratio, however. This would give you the number to plug into a calculator.

Awesome, thank you

[wolfsden]

Originally Posted by geezer#99

Look around here for calculators.
https://transmissioncenter.net/shop/

To figure out what rear gears you have, read here.
https://www.google.ca/webhp?sourceid...d+gear+ratio&*

Great info. I will try it Thanks!

[geezer#99]

Your needle bouncing could be just old grease inside the cable. You might need to pull the inner cable out and clean it.

[wolfsden]

Originally Posted by geezer#99

Your needle bouncing could be just old grease inside the cable. You might need to pull the inner cable out and clean it.

It's a brand-new speedo cable that I replaced

[Wrenchbender Ret]

If you are reading fast you need a larger driven gear . Try a 44 tooth gear & check the odometer with the mile markers on the freeway.

[wolfsden]

Originally Posted by Wrenchbender Ret

If you are reading fast you need a larger driven gear . Try a 44 tooth gear & check the odometer with the mile markers on the freeway.

My Speedometer is reading faster than what I am actually traveling so I believe I need A smaller driven gear.
I did the calculation today, I found out that I have a 3.73 rear my tire size is 29.3 so I need a 39 tooth driven gear I have a 42 tooth in there now.

[Wrenchbender Ret]

Its going to read faster yet with a 39 tooth gear.