With the change to an M20 this summer Im wondering if I should change rear gears. I have 3.42 in a 71+ rear end Yesterday on the highway at 65 mph I was at 3000 rpm.... too much? A friend has offered me 3.08s. Im no expert but I think this would put me at approx 2600 RPM ? Tires on the back are 295 50 15 Suggestions? Thoughts? What did a factory big block 4 speed come with?
Ran some numbers through Wallace Racing. 3.08 gear would be about 2550 rpm, 2.93 would be about 2400 rpm. Changing rear tire to 295/60-15 would be about 2600 rpm. Hope this helps.
its all about what youre comfortable with. Personally, I like the 3.08s with a 3 speed trans. They give good wheel speed for smokey burnouts too......
I ran 3.08's in my Skylark for a while with the manual trans and it was "okay". It all depends on what kind of driving you do most with the car. If it's mostly highways, cruising and the occasional burnout, then 3.08's are pretty good. You will certainly feel the difference off the line with the 3.08's. I went from 3.08 to 3.42 and it was pretty night and day IMO. The highway with 3.08's is pretty nice though, especially without overdrive. I towed a small utility trailer with an engine and trans on it for several hundred miles (from Phoenix) and the nice thing about the 3.08's was being able to leave the car in 4th gear on the highway*. IMO, 3,000rpm is not too much. My T-bird with a 428 FE engine runs right at 3,000rpm at 70mph, and my Centurion is not far behind it at like 2,700-2,800rpm (both pretty much stock cars). If you want to count it, my Firebird with the L6, with 4.56 gears and an overdrive runs between 2,800-3,000rpm down the highway. *Part of my issue with the 3.08's is I ran into pinging/detonation issues from the extra loading up of the engine at lower rpm (namely 1st gear). Your engine combo may not have that issue.
Curtis. The '70 GS455 came with a 3.42:1 ratio as standard with a manual transmission. Stage 1 cars got the 3.64:1 Limited slip with no air conditioning, and 3.42:1 with. Using the following formula, you can calculate theoretical RPM @60mph. 63360 / pi / tire diameter x final drive ratio = RPM. In your case - 63360 / 3.1416 / 26.6 x 3.42 = 2593 RPM. Further 2593 / 60 x 65 = 2809 RPM. It sounds like your RPM is about 200 higher than this calculation. If you feel that is uncomfortable at highway speeds a 3.08 would result in the following: 63360 / 301416 / 26.6 x 3.08 = 2335. Further 2335 / 60 x 65 = 2530 RPM. Richmond makes a 3.23:1 ratio for the 8.5, and may be a good compromise with an RPM of 2653 @ 65 MPH. Some people experience noise with Richmond gear sets though.
Unless you are married to the rear tire combo, you should be able to tune out some RPM with tires. Maybe you can borrow some taller tires to see how it goes. I used to run 255-70-15 radials on the back of my car with 4.10 gears and it was just about 3000 at 60. I've got 3.08 gears in my 350 GS with an M21 and it is a dog off the line but is about 25MPH per 1,000RPM in 4th with G70-14 radial tires (225-70-14). I need to find me a M20!
Tires are brand new so I'm kinda married lol Mpg or economy is not my concern. If 3000 rpm is normal I'm ok with that. Buzzing along for an hour at 3k seemed loud. With the th400 I don't remember it being that high though I knew the overall 1:1 is the same on both.
When I leave town, I buzz along around 3,000 for at least 4 hours, and usually 7.5 hours. Easy work. It's a personal preference thing, especially when not worried about fuel economy. If anything, the TH400 was probably 200-300rpm higher at the same speed, unless of course the tire diameter changed with the 4-speed conversion.
I drove my car with 4.10 gears for 4 years every day as my daily driver in college. It was about 12,000 miles a year. It went to Florida once, to N. Carolina a few times and any place I needed to go. It hummed right along at 3500@70MPH. It really didn't hurt the gas mileage. I got 16MPG on the highway if I kept it right at 70. It is actually less stress on the motor spinning faster than gearing it too high and lugging it along.
I attended a seminar with lead Stage-1 engineer from Buick Dennis Manner. Someone asked the question how high should you rev on the highway. He said you can run all day 3000-3500 RPM all day long. That is what they were meant to do. Cars in the day just do not rev low like the newer cars. They ran the 455's in the labs for months like that and they never stopped. I am not going to argue with him. This is assuming your engine is sound. I would also check the oil often.
If the 3.08s are free, you could try them. But something tells me you will be happier with the 3.42 in the end.
If most of your driving is around town stay with the 3.42's. Much easier on the clutch taking off from a stop. One thing to consider is the overall 1st gear ratio. M-21 2.22 x 3.42 = 7.59 M-20 2.52 x 3.42 = 8.62 M-20 2.52 x 3.08 = 7.76 As you can see changing to 3.08's will be a large change taking off from a stop but slightly better than the original factory setup. If this was a 350 car I would suggest staying with the 3.42's. Being a 455 car the decision is a toss up. I have used most every rear gear available from 2.56 to 4.11 with the M-20 and M-21 in my 70 455 4speed GS from 2.56 to 4.11. The original factory setup is a good compromise for all around driving conditions and the one I am currently running. The M-20 with 3.42 gears is the one I liked the best.
