gear ratio id

[Bunkerhillbilly]

How do I figure out the gear ratio on a 9" third member with no tag?

[HIO Silver]

Put the rear up on jackstands, the trans in neutral, then turn a rear wheel one rotation while at the same time observing how many times the driveshaft turns.

[Bunkerhillbilly]

Third member is on the bench. It's got 31 ring gear teeth and 10 pinion teeth

[ultraranger]

Third member is on the bench. It's got 31 ring gear teeth and 10 pinion teeth

31 ring gear teeth ÷ by 10 pinion gear teeth = a 3.10:1 ratio. 35 ring gear teeth and 10 pinion gear teeth would be a 3.50 ratio

I'm not aware of any 3.10 ratios in the Bumpside trucks. The only Ford 3.10s in a 9-inch I'm aware of were in the really older model Fords --like in the '57-'59 era. However, any Ford 9-inch 3rd member can be installed in any Ford 9-inch rear end housing, so long as the spline count of the axles for the rear end housing matches the spline count of the axle gears inside the differential of the 3rd member.

[HIO Silver]

Third member is on the bench. It's got 31 ring gear teeth and 10 pinion teeth

Well that would have been helpful to know....

Then the definition of "ratio" is self explanatory.

[Bunkerhillbilly]

Thanks. It's too high geared for what I want. My buddy said he take it back. No harm done. Now I know how to figure out ratios in a snap. Thanx again

[ultraranger]

Any of the ratios listed for the Bumpsides were optionally available in any of the trucks. However, and generally speaking, six cylinder-powered trucks GENERALLY will have lower ratios than V-8 powered trucks.

If you look on the drivers side door tag under the "Axle" heading, you'll see a ratio code. This will most likely be a two digit code. There were various ratios available but, the three most common Ford 9-inch axle codes you'll see on the door tag will be 08, 09 or 17. Two digits means the differential is 'open' (single track). 08 would be a 3.50 ratio. 09 would be a 3.70 and 17 would be a 3.25 ratio.

If the axle code is an alpha-numeric (B9, for example), the rear end will have a limited-slip differential. A B9 would be a 28-spline Traction-Lok differential with 3.50 gears.

--Note: the warranty tag on the door could be missing, the door could have been replaced, someone could have swapped out the 3rd member in the rear end or, someone could have swapped out the entire rear end of a potential donor truck.