Since I am installing a posi rear with 3.08 gears vs. the stock 2.28 in my 1979 Deville, my speedometer will be off. What needs to be done to correct it for the new rear gears? I assume the speedo drive gear on the back of the transmission needs to be changed to compensate, but I'm not sure what to get, or where to get one.
Any suggestions?:thumbsup:

Well in the 77 DeVilles the 3.08:1 gearing was an option so you could probly get that diffrent drive gear from your local parts store.

Great idea, I didn't think of that! Thanks!

not a sure thing - both cars had to have the same tire size, and the same driving gear (the gear on the trans output shaft) for that to work.

http://www.tciauto.com/tech_info/speedo_gears.htm
Check that link. If you scroll down on the page, you can find the info to ID the driving gear in your trans by the color of the plastic and find how many teeth it has. Then follow their equation to find the proper driven gear.

I'll check that out. Thanks!
I would assume the 77-79 Devilles used the same transmission and tire size, with just different rear end ratios. But I'll look into it.

The tire size I believe should be the same, but I wouldn't assume the drive gear in the speedo is the same. There are a bunch of different gears, and since they are just a plastic piece that slips on, the factory might have gone to a different drive tooth count to get the speedo as close to accurate as possible.
By just changing the driven gear, it is likely that you can get within a few %, under 5% for sure. Changing the drive gear is a bit of a pain, so I'd be happy and leave it at that. But, the factory guys might have changed the drive tooth count to get it just that much closer.

Unless they changed to a different kind of gear or something changed because of the transmission just get the right gear for the rear end that you have and it should work properly.