Gear Ratio and Carrier

[Mike C]

Your tires are approximately 30" tall. (285/25.4=11.2" 11.2*.55=6.17" sidewall height *2+18=30.344) Stock tires were approximately 28 tall. That is why your speedo is off. 2*pi*r for your new tire is 94" while the OEM tire would be 88". The new tire rolls 6" further for every rev or 7% which will be approximately what your speedo is off. Assuming the 3.08 is your original gear.

The 3.08 is a 3 series carrier which would be good up to 3.55 gears. 3.73 and up would be 4 series, in a truck. Different break point on gears in cars.

[smoky02]

Originally Posted by Mike C

Your tires are approximately 30" tall. (285/25.4=11.2" 11.2*.55=6.17" sidewall height *2+18=30.344) Stock tires were approximately 28 tall. That is why your speedo is off. 2*pi*r for your new tire is 94" while the OEM tire would be 88". The new tire rolls 6" further for every rev or 7% which will be approximately what your speedo is off. Assuming the 3.08 is your original gear.

The 3.08 is a 3 series carrier which would be good up to 3.55 gears. 3.73 and up would be 4 series, in a truck. Different break point on gears in cars.

So that means that if I want to run a gear higher than 3.55 I would have to change out my entire rear end? Or can you buy 4.10s for a series 3 rear end?

[texnician01]

Smokey, you will need to either find a series 4 carrier if you want to use 4.10 gears or buy a set of thick gears (very common) for use on your series 3 carrier. I think I may have an open series 4 but I would have to look, if you are interested. I've never used a thick set of gears but I have to wonder about strengh, does putting that thicker gear in, moving the force away from the flange add extra stress? Anyone ever used a set of these gears. You can get thick gears about anywhere you can get normal gears, a quick search led me to several places to get them but they are about $50 higher because of the extra metal.

Good luck

[texnician01]

Just noticed you have a posi, the best thing then will most likely be the thick gears. When I was looking for a posi it proved to be quite a daunting task, people either didn't want to part with them or they didn't have any.

Also, you will not have to change the whole rearend, just the carrier. The housings and axles are all the same except for length. Your truck would have originally had a narrow axle, the 70 and up trucks had an axle 1 1/2 inches wider, 3/4 on each side. The carriers are interchangeable for any truck 12 bolt and they used them up into the 80's. When changing a carrier and gears, it's not a bad time to rebuild the whole rearend with minimal extra cost.