View Full Version : Rear Axle: Best Rear Gears With OD And V8

09-25-2017, 07:59 PM
A few years ago, I installed a 700R transmission in the wife's 63GT. The first rear end I tried was a 3.07. It got 22-23 MPG on a trip to and from South Bend, but I felt it was a lacking in power. So next I installed a 3.31, and it was OK, but still a bit lacking in power. Most recently, I installed a 3.54, and it seems to be the best match of the three. In TC lockup, final output drive is 2.48, and it runs about 61-62 MPH at 2000 RPM. Overall, whether city, country, or on the interstate, I must say the 3.54 seems the best match.

In the 62GT, with T85, a 3.54 has been fitting the bill nicely now for over 300,000 miles. In high gear OD, final output drive is 2.55.

09-25-2017, 11:14 PM
I admit, which is probably no surprise to anyone that I have never owned a Stude. V8 with a GM Transmission.

However I always thought that with the reported very Low, Low Gear start in the THM's (a little Higher in a 2004R: 2.74 vs 3.06), that a 289 4V would have good "Power" with one of these very common 700R4 GM Trannys with a High Geared Axle Ratio like 3.07 or 3.31.

I guess what I am forgetting is that at speed with wind and hills there might not be "enough'" in O.D. requiring too many downshifts and wasted Fuel.

Is that about how you would describe your lack of "Power" Joe?

My only experience DRIVING one of these, was my Son's '59 Lark 4 Door with a fully "Built" roller Camed small block 400 Chev., a 700R4 and only a 3.07 Ratio.
That Car could light up the tires for blocks if you stayed on it, and would easily spin out on turns and try to "flip ends" with too much throttle and flat Fly to over 120 MPH, it sure was not short on Gear or power, but not a comparison to a 225 to 240 HP 289.

09-25-2017, 11:32 PM
Many years ago - like 1965 - the second '57 Silver Hawk I ever owned, was a straight 3 speed and thus had a 3.54 rear end. I changed the transmission to an overdrive, and what a perfect combination. I drove a lot on the I-5 freeway and it had a comfortable cruising engine rpm range, and had great gas mileage, even with the '59 Pontiac AFB carburetor.
So, yes, a 3.54 ratio is great!

09-26-2017, 10:31 AM
In some regards I have a similar setup like the original you presented. A 700R4 with 3.07 gears. My tire size is 225-60-16" and I'm guessing "close enough" to the diameter of the 205-70 (or 75)-15" tires most seem to run. The math has 65 MPH at just under 1,700 RPM. As mentioned above I don't have much issue with getting the car moving with the 3.06 1st gear. Nor on the highway do I find the final drive (4th gear, convertor locked up) too much for the hills I need to climb. My difference maker is a 350 Chevy engine from an '85 Corvette. The injection is gone, replaced with a 650 Edelbrock but the rest is stock. And the only difference between that Corvette engine and any other SBC 350 is a marginally hotter cam. My understanding is that that engine was built for torque and for sure it falls flat at 5,000 RPM. But 61 C.I. over a Stude 289 is a 20% increase.

My greater issue is in town driving. I've had to resort to just leaving the car in 3rd (1 to 1) for city streets. The ratio of 4th gear was OK at 45 MPH, but the lock up of the convertor (which is necessary) made things unpleasant. You are likely right that the 3.54 ratio is best with your application. I find it ironic that on the Gen. 3 Camaro's they went as far as 3.73 on the performance end and a seemingly ridiculous 2.73 on the mileage end (with 16" tire no less). I recall reading that freeway speeds were sub 1,500 RPM.