308 gears with a 4 speed

[John G.]

Re: 308 gears with a 4 speed

I plan to do this exact swap (4:11 to 3:08) in the next 1-2 months, also for long road trips out of state to visit the grandkids, etc. . . With M-20 first gear starts should be a little easier. If not for already having a 3:08 carrier assembly ready to go, a 5 speed OD conversion would have been real tempting. Except for the freeway, the 4:11's have been great for everything else.

[Joe L.]

Re: 308 gears with a 4 speed

Men thanks for the Replies,

Joe are the pinion flanges interchangeable, or are the splines different on the pinion shafts? Yes this gear ratio has been around before "68" but was introduced to the Corvette in "68" with the Turbo 400 as a stock set of gears. I would be Happy to
be at 2500 RPM's at 70 MPH ! Thats my goal, I don't want to Re-ring this engine after a 2000 mile road trip.

Jeff-----


Pinion gear splines are the same for All 1965-79 Corvettes. So, the flanges are interchangeable. 1963-64 units use a different spline and require a specific flange.

[Monte M.]

Re: 308 gears with a 4 speed

Jeff,
It sounds like the original Idea was you plan on using on the open road. I have had many different gear sets in many different cars. Depending on what I am using the car for. My cars came with 3:08's from the factory. Using them around town, I do not enjoy the cars nearly as much with the 3:08. But, when I plan on doing any kind of real destination driving, I for sure throw the 3:08's back in it.
It is such an easy swap that, IMO, it is well worth having a set of 3:08's around.
If you are near me, I have an extra 3:08 set-up you can throw in your car to see if you like it. It not, nothing lost except a little time.
Personally, after you get used to it on the freeway, you will think it is the way to go.
For sure, do not part with your other diff though. If you are just going to stay local for a summer, throw the other diff back in. A lot more fun to drive.

Best of luck,

PM me if you like

[Monte M.]

Re: 308 gears with a 4 speed

Jeff,
It sounds like the original Idea was you plan on using on the open road. I have had many different gear sets in many different cars. Depending on what I am using the car for. My cars came with 3:08's from the factory. Using them around town, I do not enjoy the cars nearly as much with the 3:08. But, when I plan on doing any kind of real destination driving, I for sure throw the 3:08's back in it.
It is such an easy swap that, IMO, it is well worth having a set of 3:08's around.
If you are near me, I have an extra 3:08 set-up you can throw in your car to see if you like it. It not, nothing lost except a little time.
Personally, after you get used to it on the freeway, you will think it is the way to go.
For sure, do not part with your other diff though. If you are just going to stay local for a summer, throw the other diff back in. A lot more fun to drive.

Best of luck,

PM me if you like

[Jim T.]

Re: 308 gears with a 4 speed

Jeff my first Corvette, 1964 327/300 factory air conditioned convertible with factory installed 3:08 gears. Really liked having the 3:08 rear gears especially when I drove it from San Diego-Dallas-Glynco, GA and back to Dallas in summer of 1969. Gas mileage was good, don't remember the actual mileage. Best mileage on my 68 327/350 M21 and 3:70 on a road trip of 183 miles at 55-60 mph was 22.04 mpg in 1989. Rest of trip was 19.4-20 mpg. Holley 6210 double pumper carb in use.
Have always liked the long distance trip mileage my 1970 350/300 with factory airconditioning turbo 400 gets with 3:08 gears.
Stopping at the stop sign near my home my 68 has to take off from an incline. The engine is cold since the stop sign is less than a block from the driveway. Definite different technique is used to get moving without slipping the clutch only just a little.
My 1996 with LT4 does not lug in 6th gear at 70 mph and the tack shows about 1,700 rpms. Best mileage ever 30 mpg without using air conditioning.

[Michael M.]

Re: 308 gears with a 4 speed

Hi Jeff,

My 66 L79 has 3.08 gears with an M21. It's great on the highway, especially with side exhaust (not so loud at lower revs) and absolutely fine around town. It takes a bit of slipping the clutch/getting the revs up when taking off, you get used to it really quickly. Not normally a problem, but facing uphill stopped on a grade can be a challenge now and again. I'm not sure what my car had from the factory for gears, I can't read the code on the diff carrier, but the speedo is calibrated pretty close, I think my tires throw it off just a bit (it reads a little high, maybe 3-4 mph). I intend to swap out the cr for a wr gear set next winter, I'll drop the trans when I get the trailing arms, etc.. rebuilt, but for now it's fine. I think you'll love it, unless you like 1/4 mile runs.

Mike

[Steven B.]

Re: 308 gears with a 4 speed

Jeff, getting rolling from a stop or low speed is impacted by torque as you know. What does your engine torque at on the dyno?

[Paul B.]

Re: 308 gears with a 4 speed

Gearheads,

Were getting ready for a road trip and I was thinking of swapping the rear diff to a 308 gear ratio. I have been running a 4:11 forever with High RPM's. The 327-430HP and a m-21 close ratio will be at ease at 70MPH. Hopeing to stay out of the Gas stations and gain some miles per gallon with this set-up. Whats your thoughts on the RPM's at 70 MPH and the possibility of Lugging at low RPM's. This rear gear was first introduced in 1968's Turbo 400. Can I swap this rear diff. without any alterations to the Driveshaft?

...I did this 15yrs or so ago to my '68. You stated "430hp"? That's an L-88. Sure you don't have one of those? When I did this to my L-79 327/350hp M-21 car, mine originally was a "3.70:1" rear end car. When I had the swap done(I removed the rear end myself and a reputable shop did the swap of gears)there was talk that it could only be done to a "3" series carrier, in other words, in order to have a 3.08 installed, the original rear end had to be a 3.36, 3.55 or 3.70 geared rear end. Not sure on this, maybe someone else can chime in? Another thing, the M-21 close ratio trans has a taller(higher)first gear than an M-20 wide ratio trans does. Not by that much, but it does. Usually I just have to be a little bit more slower and gentler in letting the clutch out. I dropped appx over 550rpms going from a 3.70 to a 3.08. In your case, it's be just a tad over 25% reduction on the rpms. Driveshaft turns 4.11 times with every revolution of the rear tire, so you'd be going over to 3.08 revolutions of the driveshaft per every revolution of the tire.