[bj1160]

I have changed out my rear end from a 3.08 to a 3.73 and have 255/60/15 tires. The original drive gear was 39 teeth what do I need to change to? I don't know how much it is off because I am just now putting the car together and wanted to fix it while the tranny was out.

Brian

[bj1160]

I have changed out my rear end from a 3.08 to a 3.73 and have 255/60/15 tires. The original drive gear was 39 teeth what do I need to change to? I don't know how much it is off because I am just now putting the car together and wanted to fix it while the tranny was out.

Brian

[STXJudge]

Assuming your drive gear was a 7 tooth gear, you need a 19 tooth driven gear. My drive gear is 7, but I've got an odd transmission, so here it what I used to figure that out in case yours is different.

http://www.tciauto.com/tech_info/speedo_gears.htm

[Shadowjack]

A 39-tooth gear is from a TH400 and the sizes don't go down to to 19, which is a manual piece.
Street tire diameters don't REALLY vary much from 26", so we'll guess the same as stock.
Each step in rear gear is about 5 percent, so going from 3.08 to 3.73 is 4 jumps: the driveshaft will turn 20 percent faster.
Slowing down the speedo cable by 20 percent takes a driven gear with 8 more teeth: 47. Unfortunately, the biggest gear available is a 45, which by the way takes a different adapter sleeve.
The factory used conversion adapters on the cable to finish the correction. You can get stuff like this from speedo specialists, but I don't know of any off the top of my head.
Of course, if your tire is bigger or littler than stock, it will change the ratio as a percentage as well.

[form406]

i need to fix mine too...my rpm's are about 50% of my speed until around 50 mph...then the faster i go it seems about 15-20 mph too fast...

[Otto Tharp]

My days at the Cadillac dealership with Turbo 400 we found that a one tooth difference equalled about 2.5 mph difference around 50 mph. Fewer teeth spins the speedo faster and more teeth slows it down. Figure out how far you are off and go from there. It may not be perfect at all speeds. The speedometer sleeve that the gear goes in will be marked with 4 different numbers for the 4 gears that will fit that sleeve. Ex. 35,36,37,38 will take any of those gears. As you get to larger gears they had to offset the center of the sleeve to compensate for that. Take a sleeve and look at the center point and you'll see what I mean. I believe they may have had different output shaft gears, but I can't remember. I recently still bought some gears. Remember to replace the rubber oring on the outer part of the sleeve and get a new seal for the shaft part of the gear. It is held in by a small clip that you can remove with a pocket screwdriver. The new seal willinstall nicely with a 3/8 socket and put the clip back in on top. If the shaft on the gear is worn you can double stack another seal on top of that one. Sorry for the long message, but thought I would give you everthing I can.

[beemergary]

Call TCI transmissions they will fix you up. measure the circumfrence of your tire. I can't find the formula for drive and driven gears. But they can be hard to find, also you may need a different speedo gear housing. Lots of 350 stuff around. Just call TCI

[beemergary]

I found it. #drive teeth X axle ratio X tire rev.per-mile divided by 1001=#driven teeth Drive gears 7 8 15 16 17 18 19 20 21 Driven gears 16 17 18 19 20 21 22 34 35 36 37-45 Good Luck