Hey everyone i'm new here but need a little help. I had to replace the rear rear in my 89 fullsize blazer4x4 , it had a 10 bolt 3.08 gears,the rear I put in was a 12 bolt 3.40 gears.Thats what I had,would like to know if the different gears will make much difference on my speedometer and if I should take the front driveshaft off for nowbecause of the gears.Now I know I can't use my 4 wheel drive cause of the diff gears till I change the front rear but needed to fix so I could drive it.The rear I put in had the numbers GM 82 & 3896839N stamped on the center housing anyone know what this rear came out of since I don't think it was stock in the 83 GMC4X4 I took it out of ? All answers would be very helpful. Thanks

 

From those numbers, it appears to be the last of the GM Corp 12-bolt axles from a 1982 application. Not much else is in those numbers. There may have been a tag on it at one point in time to give further information.

Going from 3.08:1 to 3.42:1 (the ratio you most likely have) would impact your speedometer by a factor of 11% if that is the only change. With stock tire size, this will cause your speedo to read slow, with an actual ground speed of 53.4MPH corresponding to a speedometer reading of 60MPH. If you have changed the tire size, then that must be factored in as well.

And as you well know, you CAN NOT drive the vehicle in 4wd with this mismatch in ratios.

 

Thanks for the answer but the ring gear is stamped with 10:34 numbers and I'm running 31x10.50x15 tires. Do you think I should remove the front drive shaft ?

 

Have you counted the teeth on the pinion and ring gears?

So long as you keep the transfer case in 2HI, you should have no problems. If you have locking hubs up front, leave them unlocked and you are sure to have no problems.

As far as the tires... It now depends on what you had for the original size. I show either 225/75R15 (28.3" outside diameter) or 235/75R15 (28.9" outside diameter) as being available stock sizes. So, your speedo will be off by +7.3% (235/75R15) or +9.2% (225/75R15). The gear ratio difference of -11% could actually offset the tire size difference quite well. Well enough as to not really worry so much about it as it is still conservative. If your stock tire size was 235/75R15, then you may still want to correct the speedo (which I think is electronic...).

 

Thanks for the help. Keep the shinny side up and the dirty side down and have a good one.