1972 Cutlass, TH400. I'm changing rear end gears from the stock 2.73 to 3.08's. Currently, it has the stock 18 tooth drive gear and a red 37 tooth driven gear. According to a chart I found online I THINK all I need to do is go to a 42 tooth Green driven, which also requires replacing the driven gear housing to one that accommodates 40+ tooth gears. I'm looking for verification on this, hoping someone else who has swapped rear end ratios has already figured this out.

thank you

 

HEY THERE
I would just leave the speedo gear that came with the 2.73 in the car. there will be "virtually" no difference in what the Olds speedometer reads
That is unless you also changed the tire size as well

All the Best
brian

 

Really? I've re-geared a lot of cars over the years and I've always had to change out the speedo gears to correct the reading. Are you suggesting that because the change from 2.73 to 3.08 is minor?

 

Yes

IF it were a drastic ratio difference I would tell you to swap gears etc for speedo
Check speedo with GPS

IF all was accurate and working with the speedometer before it will only read slightly different with 308 gear from 273

are you running factory size tires??

 

yep, stock size tires

 

Upon further research and calculations, i determined that my speedometer would read 8mph high with the gear change. so to be travelling 60mph I would need to run with my speedo at 68mph. It turns out in my drawer of speedo-gears that I've collected over the years I have the gear, just need the housing which isn't expensive. Got my diff rebuilt this weekend, gears set up with a really nice pattern and did new wheel bearings and seals at the same time. Now to wait until spring to actually drive it.

 

Who did your rear gears for you? Can I ask what it cost you? I am set to go to a shop nearby to have this done. What part of Minn?
Thanks!

 

There is an 11% difference between 2.73 and 3.08.

 

I did them myself. I've set up several of the GM corporate 8.5" rears over many years. My first few were definitely learning experiences, but once you figure it out and have all the right tools it really just takes patience. I spend at least a couple days taking my time to do it right. Lots of trial & error to get the right pattern. Plus I have a lift in my shop which makes it a heckuva lot easier on my 53 year old body compared to laying underneath the car like I first did them in my 20's.