Discussion Forum - Northstar Chevelle Club

jim larson · 2K+ Club

Changing rear axel ratio question

67ss · 1K+ Club

Unless you are doing it on a hoist it is much easier to have rear axle out of the car.

jim larson · 2K+ Club

I was think along your lines Chris. Especially for an old guy like me that ring gear and carrier are kind of heavy, much easier to remove and change the shims for the carrier and pinion to get the correct backlash and gear set. No hoist here.

Bobs_Place · Vice President

I replaced pinon Brgs with the diff assy. in the car. BUT on the hoist!

You will spend some time to R & R the housing, (on your back) and that not fun either.

dashboard · 2K+ Club

What gears are you going from to Jim?

jim larson · 2K+ Club

dashboard wrote:
What gears are you going from to Jim?

Currently I have the M20, so first is 2.52:1 then 1.88:1, 1.46:1, and 1:1. My current ratio 3.31:1. So that gives me wheel ratios of 8.34, 6.22, 4.83 and 3.31 per engine revolution.

When I look at the M21 ratios and a 3.73 gear, I get 8.20, 6.11, 4.77, and 3.73.

So currently in 1st though 3rd gear, I am lower than the M21 with the recommended 3.73 gear ratio. If I go the the 3.07 gear ratio, I would be a little higher in 1st though 3rd. Gving me 7.74, 5.77, 4.48 respectively and 3.07 in drive. All lowering rpm by about 7% and increasing milage by about 7%.

I have the gear, so only cost is fluid and a cover seal, with my labor which isn't worth much and would keep me out of the house during the day for a few days.

dashboard · 2K+ Club

Lot's of numbers there Jim. Looks like the big ones are the 1:1 forth gear numbers; that's were you'll spend most of your time. Ever think five speed? I'll never go back, my M22 sits in it case on the shelf.

OscarZ · Mega Poster

Jim why are you wanting the change? Going racing? I have 3.73 gears with a 28" tall tire and and turn about 3200 RPM at 65 MPH. Most of the events I would like to attend include 70 mph interstate or 65 mph highway included in the best route and that gear without overdrive is a bit of a PITA. Plan to go with a 200R4 as soon as I can swing it and all will be good! Don't get rid of those 3.31. I regret letting mine go now that I live farther away from most things.

jim larson · 2K+ Club

I still have the 14" rims, so the tire diameter is 26.5". Thus with the 3.31 I am turning about 2520rpm @60mph. That would drop to 2337rpm @ 60 mph if I go with the 3.07 rear.

No racing, just better milage and less rpms. I think the total cost would be less than $100 as compared to the cost of a new 5 sp.

Jon H · 1K+ Club

jim larson wrote:
I still have the 14" rims, so the tire diameter is 26.5". Thus with the 3.31 I am turning about 2520rpm @60mph. That would drop to 2337rpm @ 60 mph if I go with the 3.07 rear.
No racing, just better milage and less rpms. I think the total cost would be less than $100 as compared to the cost of a new 5 sp.

Seems like a lot of hassle, time and money for 183 RPM. I question whether you will even notice the difference. If you had to go in to change something else and did it because you were there is one thing. Sometimes it is better to leave well enough alone. Just saying...

Lost in the 60s · President

Jon H wrote:
jim larson wrote:
I still have the 14" rims, so the tire diameter is 26.5". Thus with the 3.31 I am turning about 2520rpm @60mph. That would drop to 2337rpm @ 60 mph if I go with the 3.07 rear.
No racing, just better mileage and less rpms. I think the total cost would be less than $100 as compared to the cost of a new 5 sp.
Seems like a lot of hassle, time and money for 183 RPM. I question whether you will even notice the difference. If you had to go in to change something else and did it because you were there is one thing. Sometimes it is better to leave well enough alone. Just saying...

I agree with that...and you won't notice much change in fuel consumption either. Maybe a couple tenths more per gallon.

If you're really looking to improve driveability and mileage, an overdrive trans or gear vendors overdrive unit are the only options that will produce enough of a change to notice or benefit from.

SShink · 3K+ Club

OscarZ wrote:
Don't get rid of those 3.31. I regret letting mine go now that I live farther away from most things.

Sorry to hear that Mike since the ones you gave me ended up in the green '72 Chevelle that's now somewhere in Texas when I sold it during the days I was flipping Chevelle's every 18 months... The 3.31's were a great overall gear, especially with the M20 and 454 that I put in that one.