Rear gear change

[SmallHurst]

Lets see, '72 Olds having the 8.5" GM 10 bolt...lots of choices indeed.

For the torque that you have in that Olds a 3.42 or a 3.55 would make for a real runner. Mine had the 3.23s in it and on the drag strip, they ran within .02 of the 3.91's. If I could go back, I would try to find a set of 3.42's but, Richmond does not make that for the 'O' axle. 3.73's will make it a little buzzy, but you can still run down the highway.

[beater68427]

[ QUOTE ]
Lets see, '72 Olds having the 8.5" GM 10 bolt...lots of choices indeed.

For the torque that you have in that Olds a 3.42 or a 3.55 would make for a real runner. Mine had the 3.23s in it and on the drag strip, they ran within .02 of the 3.91's. If I could go back, I would try to find a set of 3.42's but, Richmond does not make that for the 'O' axle. 3.73's will make it a little buzzy, but you can still run down the highway.

[/ QUOTE ]

I agree with you, what the original poster should do is figure the cruise speed he likes on the freeway, and figure that with 3:73's what RPM he would be at with tire size lets say in the area of 3000rpm then take his car in current form put it in 3rd gear and drive on the freeway for 10 miles or so at 3000rpm see if he cars for the running rpm of the car for a time period, then do the same with a 3:42 calculation, and see what typical cruise rpm he is comfortable with. also a typical Olds especially if it has alot of miles on it likes to fill the valvecovers up with oil at a higher sustained RPM a typical problem of them, it is worse if they are a older engine the two return holes in the back of the head are quite small and sluge up at the base leaving only .125 or so of oil return. I would recommend restrictor push rods with plans of such a rpm increase, If the engine is rebuilt with a HV oil pump and restrictors have not been installed in the rebuild I would highly recommend them as well. If you need info on stuff like that you need to talk to Mondello. http://www.mondellotwister.com/ I have had quite a few Oldsmobiles, and driving characteristics are quite different than a BBC on cruising RPM. This is my current 72 442 under a long rebuild... Good Luck with the change over!

[JOZW30]

Engine has about 3k miles since rebuild with NOS W31 cam and higher than stock compression pistons (not flat top, but lower cc).

M20 was rebuilt last spring.

I think I'm going with the 3.73's and worse case, I can travel country roads at 50 MPH if it's that bad on the freeway.

Calculators say I'll be between 3100 & 3200 RPM @ 65 MPH with my tires. 3.42's would lower it by roughly 200 RPM.

Appreciate all of the input!

Joe

[m22mike]

Joe
If you can cruise @ those RPM'S you will will be OK I think.
My L78 Camaro, M22 with 4.10's and F70 14 tires is at 3800 R's at 62 MPH.
Now I sure would not take off on a power tour, but I sure as heck ain't given up those 4.10's
You need to do this at times

I do make some 20 to 40 mile trips on the I state's and 60 seems to be my comfort zone. Good luck.

Mike