4:11 rear gear swap

[Wally A.]

Looking for someone who would be recommended to do a gear change for my 65
from its current 4:11 , need to do something for highway use going to a 3:36
central florida area, unit is out of the car... any input would be appreciated.

[Ed S.]

Re: 4:11 rear gear swap

What engine is in your car? Is it stock or has it been modified? If modified, how so?

[Bill B.]

Re: 4:11 rear gear swap

I highly recommend NCRS member Gary Ramadei in CT. He is a subject matter expert in differentials and a frequent contributor on this Tech Board discussion site.

[Wally A.]

Re: 4:11 rear gear swap

Ed, L76 with a close ratio, Fuel is killing me and 3500 RPM is killing the motor as well. I do a lot of hwy. driving
and 3:36 is the way im going, looking for a reputable guy in central Florida to do the job, the unit is out.

[James W.]

Re: 4:11 rear gear swap

I had Gary Ramadei rebuild my '65 differential. He does outstanding work and is on the TDB regularly.

James

[Ed S.]

Re: 4:11 rear gear swap

I know what you mean, I have a 64, L76 w/ 4:11s. But I gotta tell you..... you might not like the 3:36 gears - they are pretty tall - you may have to do a lot of clutch slipping just to get moving from a dead stop then - through 1st & 2nd it will be really sluggish. That L76 doesn't produce a lot of torque in the lower RPM ranges and that is what you need to get moving. I think others will chime in on this point. Even 3:55 is kind of tall for that engine. I believe 4:11s were an optional ratio for that engine - standard was 3:70. You might want to ask around before you take the plunge.

[James W.]

Re: 4:11 rear gear swap

I have 3.36:1 in my '64 but with the 300hp and the taller first gear in the transmission and it runs great. I have no trouble leaving a stop light. My '65 L79 has 3:70.1 and it runs great also, but does run more RPM on the interstate. I'd go with at least the 3:55.1 ratio.

James

[Kenneth B.]

Re: 4:11 rear gear swap

Am I wrong but I thought the 4-11 & 4-56 used a different carrier or case than the other ratios?

[James W.]

Re: 4:11 rear gear swap

If I remember correctly they use a 4 series carrier.

Am I wrong but I thought the 4-11 & 4-56 used a different carrier or case than the other ratios?

[Bob R.]

Re: 4:11 rear gear swap

If you change the ratio to a 3:36 your speedometer will be out and the drive gear on the transmission will be incorrect for your application. You can change from 4:11 to 3:70 and only have to change the plastic gear. Gary is a good choice for rebuilding your rear differential he is very knowledgeable and his work is excellent.

[Gary R.]

Re: 4:11 rear gear swap

Hi Guys
Thank you for your kind words and referrals.

In respect to a 65 411 diff. Some things need to be known before you pull the plug on working on one. If the car is running well but the mpg is the issues, I would really reconsider doing anything to it. A 365 solid lifter engine and CR trans work well with 370/373, 411, 456. All will suck down gas fast, it's just the way they were made.

I have been asked to do just want you want many times, even when gas was $1.50 a gallon. It wasn't a good match and one person who comes to mind disliked it so much he had 373 gears installed. If you had a WR or the tq of a BB it might be better but your set up is matched how the engineers wanted it.

Given the cost of the gears change, or to get better mpg an OD trans, I would just put in the gas and drive it. Now that's not a business format to but I am not your typical corvette vendor either. I prefer honest, quality work vs a fast buck build.

So, let's say you move forward with the gear change. 65 differentials were much better than the 63-4's but still far from perfect. There main issue is with the posi cases. This was the first year for the Eaton posi, again better than the previous Dana. They were a dual seam casting that were very prone to cracking. Once they form a crack it's just a matter of time before it moves to the cross shaft and implodes the case.

Point being, if you go there, you or whomever does the work has to know more than installing bearings, setting the lash and finishing up in a day. Those type of builds are all over the internet, they are cheap and show it, BUT it might work for your application.

If the posi is cracked it has to be replaced, don't think about drilling end hole or welding, it will still fail. Seen it way too many times. If the posi is not cracked you have 2 options- use as is or deburr, radius and polish it to help prevent cracks. Nothing is 100% but the polishing does make a difference. I don't think you are drag racing, as you have the better setup, so you won't need some of the HD mods to strengthen the diff. I would tune the posi vs a stock setup and fit the caps to 001" rock at a minimum. Also, cheap insurance is a set of ARP ring gear bolts. Many of the "vendor" rebuilds I get in to fix are nothing more than a set of bearings and a paint job. Ring gear bolts sometimes are not even touched. Others use the "kit" bolts which are not shoulder bolts but just grade 8 fully threaded, they will work but it's a cheap out and not worth it.

All this is assuming you have an original differential to start with. Many of the C2 and C3 diff's were changed out because they were blown up street or track racing.

I am not trying to sell you anything, just point out some facts since I do this every day. Again, my recommendation is to just drive the car.

Good luck if you have any questions feel free to contact me, I'm always talking to corvette guys around the world.

Be safe

[Mark M.]

Re: 4:11 rear gear swap

Wally you could consider another rear end in 3:36 since it is the most common ratio of the 1965-1979 era. Many out there. Possibly a 3:55 which would work better with close ratio muncie. Your 4:11 carrier would have to be changed with the gears to 3 series and set up. Fresh up another good rear end would need no set up. Find a good posi 3:36 or 3:55 and store the original. If you have a local vette guy with rear ends you could save on shipping which gets pricey with a heavy rear end. There's always some on ebay but I would guess some of your local guys know someone with a few rear ends. As Gary said the Eaton carrier which started in 65 is better than the 63-64 Dana and in mid 69 the Eaton carrier casting was improved. When ever a posi carrier is out and can be inspected, the edges of the large window where the posi plates go in, should be checked for cracks around it's edges. Even new ones. The two holes in the case for the pinion gear shaft should be checked for ovaling and cracks. Most cores are good but I've pulled a number of these apart to see cracks in the window edge but it stayed together and others that didn't stay together making a mess.

[Gary R.]

Re: 4:11 rear gear swap

I have 3 brand new in box loaded Eatons on my shelf to go through and build. I don't care for the fiber clutches or the setup when I check them. The Chinese cases are not too bad and hold better tolerance than the original Eatons. The spiders are not as good as Tom Watt had made, so I mostly just use the new case and cross shaft.

[George J.]

Re: 4:11 rear gear swap

Wally,
I went from a 3:70 to a 3:36 in my '65 fuelie. Yes, it is a trade off, but it is much better on the highway. Don't worry about around town. Yes, it needs a little slipping of the clutch, but I've done it now for about 20 years, and have no plan on going back.

George

[Mark M.]

Re: 4:11 rear gear swap

If you do use another rear end, all 71-79s and 68-70 automatic cars used a larger front yoke which could use your original smaller one. That switch needs someone who knows how to set pinion bearing preload and new seals. The gm shop manuals cover all this and an experienced axle guy should be able to easily set up a good core.