308 gears with a 4 speed

[Jeff P.]

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?

[Joe L.]

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?

Jeff------


With an M-21, 3.08:1 rear gear and 327 cid engine you're going to have poor off-the-line acceleration.

I don't understand what you mean by "this rear gear was first introduced in 1968's Turbo 400". The 3.08:1 rear gear ratio existed well before 1968. Basically, 1963-79 Corvette rear gear assemblies are interchangeable. However 68-70 with THM-400 and all 1971-79 use a different pinion flange than 63-67 and 68-70 with manual transmission. A slightly shorter driveshaft is also used with 68-70 with THM-400 and all 1971-79.

[Tom P.]

Re: 308 gears with a 4 speed

EVERYTHING I own with a manual tranny has AT LEAST a 3.08 posi rear.
The 4.11, 4.56, etc. rears were great back before 1973 (Arab oil embargo and progressively higher gas prices ever since), but not any more! Also, with the 3.08 type rears, I also have wide ratio trannys. With a car as light as a Corvette and a minimum of a moderate 327 engine, a close ratio 4sp (ie M21) is not so bad (unless you're trying to haul a boat up the ramp out of the lake) and a 3.08 rear is not really too bad on take off. Once you get it rolling 5mph and the clutch fully engaged, you'll be fine. Also, with a close ratio (M21) and a 3.08 gear, the engine will wind to the moon and before you know it, you'll be doing 60mph in 1st gear. Oh ya, don't forget to change the plastic gear in the tranny tail housing. Oh, wait, that probably won't work, because if you have a 4.11 gear now then most likely you will also have to change the speedo drive gear INSIDE the tranny!

[Dan B.]

Re: 308 gears with a 4 speed

Tom hit the nail on the head for anyone that likes to drive their cars on the open roads. Thumbs up Tom!!

[Timothy B.]

Re: 308 gears with a 4 speed

That's the problem with a high performance engine and high overlap camshaft. A close ratio and 3.08 with 430hp is going to be a handful to get going. Have you thought about a 3.55 as the lowest or maybe a five speed transmission with OD.

[Terry D.]

Re: 308 gears with a 4 speed

Agree with Tom and disagree with Tim. 3:08's with a close ratio 4-speed justs takes a little getting used to, but is no means a handful. To find out the rpm's go to Keisler Engineering website, they have a speed analyzer. You plug in tire size etc and it gives you rpm with stock 4 speed and their 5 or 6 speed. If the cost of changing the rear gears is more than you are going to save on the trip, maybe a set of taller tires in the rear is your solution, a lot less money then changing gears and easy to switch back. On my 62 with stock tires and 3:08's at 65mph I would be turning 2500rpm.

[Jeff P.]

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.

[Tom P.]

Re: 308 gears with a 4 speed

My 56 with a wide ratio Muncie (M20) and a 3.08 posi turns 3000 at 76mph.
I have ZERO clue what top end is, because at 135mph it begine to float-----------------scarry as heck!!!!!!!!!

[Mike E.]

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.

It was an option for Corvette already in 1962.

[Jeff P.]

Re: 308 gears with a 4 speed

Yes and i might be wrong on this ratio combination! I thought for the Independent rear diff"s,that ratio was'nt offered till 1968 Auto's. Was a Powerglide offered in a C-2 vette?

[Bruce B.]

Re: 308 gears with a 4 speed

In my 340 HP 62 corvette, many years ago I replaced my 3.70 rear with 3.36 gears to get better gas mileage.
The 3.36 good compromize between 3.70 and 3.08 gear ratios.
But, the higher the number , the more fun to drive.

My 57 Corvette gasser project had a 5.14 rear (9.3" Olds) which I replaced with a 3.43.
After it is running I will put the 5.14 back in just for laffs.

[Stuart F.]

Re: 308 gears with a 4 speed

I have always been a 3.36 fan for both Vettes and Muscle cars. The only down side I saw was with a 1959 Biscayne with a BW T-10. When used for street (highway) racing, the long wind outs going through that gears eventually caused the roller thrust washers used on the ends of the counter shaft to weld themselves to the cast Iron case. I only saw this on the one car on which the owner did a lot of long wind outs. That didn't discourage me though and I ordered the 3.36 posi with my 63 Vette. I've done a fair amount of redline runs in first, second and third and have yet to have a reason to pull the tranny or clutch after 46k miles and nearly 50 years - but then mine is a Muncie.

Stu Fox

[Michael H.]

Re: 308 gears with a 4 speed

My new 365 HP 64 coupe came with a 3.08 diff and 2.20 close ratio. I absolutely hated it !! (car was in stock, not ordered by me)

If I had the money back then, I would have installed a 4.11 or at least a 3.70.

[Domenic T.]

Re: 308 gears with a 4 speed

Jeff,
Some here said it best, "once you get it going".
There are 2 major draw backs with that combo.
1- clutch wear, you will slip the clutch more to get it going.
2- trans wear, you will find yourself in 3rd more than you think in town and that puts the power thru the cluster not the mainshaft.

DOM

[Timothy B.]

Re: 308 gears with a 4 speed

There is a good reason GM recommended wide ratio and 3.36 and close ratio and 3.70. I have a 300hp engine in my 67 with 3.36 and starting from a stop takes very little effort, very nice to drive on the street. My brothers car is a stock L-79 with close ratio and 3.70 and it's also nice when starting from stop.

I once owned a cammed up 327 in a 1969 Chevelle SS that had a high rear and I hated it, much better with the nice 12 bolt 4.11 installed.

Jeff, tell us about the engine that's in the car.