Fun to drive rear axle change from 3.08 to ??

[Don W.]

I have a 69 coupe with 427/390HP, turbohydromatic auto trans and standard 3.08 rear axle ratio. I'm thinking about changing to something different such as 3.55, 3.70 or other suggestion to make it a little more fun to drive. Any thoughts or suggestion are appreciated. The car is a solid 2nd flight and I've taken it far as I can without spending mega $'s on paint and body (wheel flares). thank you...Don

[Patrick H.]

Re: Fun to drive rear axle change from 3.08 to ??

Do you ever drive it on the highway? If not, the 3.70 will give you a lot more punch around town.

If you ever take it on trips I suspect you'll want to stick with the 3.55, as anything lower will get you a lot more rpms at today's 70mph speeds.

[Russ S.]

Re: Fun to drive rear axle change from 3.08 to ??

Agree with Patrick.

[Mark E.]

Re: Fun to drive rear axle change from 3.08 to ??

Don,

We should trade differentials!

I wish my '70 LS-5/M20 with 3.36 gears had your 3.08 ratio. I drive it for fun around town and on road trips. It has more torque than god going through the gears, but wish it ran 1,000 RPM slower while cruising. At full throttle, 1st gear lasts maybe 1 second so I would give up some torque multiplication for longer legs.

I guess it comes down to how you use the car. My feeling is high-torque/low red line engines (e.g. L36, LS5) and highway gears are for road trips and throttle response at 2,500 RPM with the AC on. While solid lifter/high red line engines (e.g. LT-1, L71) and low gears are for hot rodding and finishing the quarter mile in top gear at red line.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

Thank you all for responses and I'm currently looking for ideas. It seems like 3.36 or 3.55 would be in fun range and I also do some highway driving. With the THM auto, it's only a 3 speed gear box from back in the day but with a good reputation. Don

[Joe L.]

Re: Fun to drive rear axle change from 3.08 to ??

I have a 69 coupe with 427/390HP, turbohydromatic auto trans and standard 3.08 rear axle ratio. I'm thinking about changing to something different such as 3.55, 3.70 or other suggestion to make it a little more fun to drive. Any thoughts or suggestion are appreciated. The car is a solid 2nd flight and I've taken it far as I can without spending mega $'s on paint and body (wheel flares). thank you...Don

Don-------


GM never installed a rear gear ratio of numerically higher than 3.08:1 with an L-36 and THM-400. I don't recommend doing it now. However, if one wants to take a chance, I wouldn't go higher than 3.36:1.

[Mark E.]

Re: Fun to drive rear axle change from 3.08 to ??

Don-------


I wouldn't go higher than 3.36:1.

Agreed. Have you driven an automatic with low gears? Bam, bam. It's over.

[Steven B.]

Re: Fun to drive rear axle change from 3.08 to ??

I would consider playing with the stall before I changed out the rear end.

Steve

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

When I purchased the car in AZ about 8 years ago, it was and is matching numbers but a complete basket case and was bought as a project. It had 4.11s in the rear; a boom box speaker all the way across behind the seats and too many other issues to list. I restored it returned the rear axle to the original 3.08:1 configuration.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

Hi Steven...can you clarify what "playing with the stall" is since I've never worked on a transmission and always just taken it to a shop...Don

[Steven B.]

Re: Fun to drive rear axle change from 3.08 to ??

What you term to be a bit more fun to drive is a little puzzling. What is the car doing or not doing that you wish to change. I don't know offhand what the stall speed for the stock configuration is for that transmission/engine, but I would guess it is moderately conservative. The converter stall is in part responsible for the behavior as the car leaves a stop and cruising (among other things like mileage). The stall speed is just the highest rpm the engine will attain with the brakes holding and keeping the car from moving (power braking). The higher the stall speed, the more torque multiplication the converter generates at stall. Torque converter selection would best be made to accommodate what you expect to do with the car. If you want a hard launch, a bit more stall might be a consideration. Personally, I would not go over about 2000-2200 stall speed for the street. You may not like a higher stall converter as it may make the transmission seem a little loose when not under hard acceleration. Another consideration is that a potential weak spot is the differential. I was not aware of this but others have indicated that they can break if pushed to hard. I'm sure others have opinions and have tried various combinations that will have more expertise with TH400/converter/engine combinations.

Steve

[Edward B.]

Re: Fun to drive rear axle change from 3.08 to ??

A "matching number" automatic with a 4.11 rear?

[Richard G.]

Re: Fun to drive rear axle change from 3.08 to ??

I am a little surprised you are unhappy with the gear ratio. With all the torque a 390 hp makes it should be happy with the 3:08's. Typically I have been unhappy when moving to options that GM didn't offer. However a gear ratio is about the only change you can make that is undetectable at judging time. I would recommend going no lower that 3:55 if you decided to change them. Keep your old gears as I am confident you might want them back when you get older or decide to go with side pipes!
Rick

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

yes, matching drive train with the rear axle modified from 3.08 to 4.11. The 4.11 was not correct and didn't match the build sheet so it when back to 3.08

[Ed S.]

Re: Fun to drive rear axle change from 3.08 to ??

Have you considered the cost of the gear change you are contemplating? I assume that you would be buying a complete serviceable carrier with the desired gear ratio (once you decide what that will be). If you have to buy one that is suspect and then have it rebuilt, oh well - you know where that is going. So now you are into at least a grand or more and all you have is a diff sitting on the floor of your garage. Then there is the installation - if you take it to a shop my guess is that they will charge you from $400 to $600 to make the swap (4 to 6 hours labor at $100 / hr). Then you will have to change the speedometer gear - not difficult or expensive. So, in order to achieve that bit of fun you are pursuing it might cost you around $1500. Not criticizing you for even thinking the swap, as we all spend a lot of $$$ on our pet hobbies - including myself, just sayin' - it is a data point worth considering.