I know this has been discussed before but I have only read where they are using 26" diameter tires. Here is my situation...my 72 Chevelle has a very mild 454 with a T350 and an almost stock 2200 stall. This is going to be my daily driver and will see lots of highway trips over 3 hours. Being that I have no OD, I want to be able to keep the RPM's down around 2500 - 2800 when doing about 70 - 75mph. The kicker is that my tire diameter is 28.9" (275/55r17)

What gear would you all recommend that:

A) Will make a nice rumble while crusing at 70 - 75

B) Will get me "ok" gas mileage (trust me I know that is far thinking)

c) Plants my but in the seat if I gun it at a light

The other thing is that I have a Vintage Air System going in as well. I won't say that this car won't see the track but probably just for fun on a test and tune night.

thanks in advance.

Jacen

3.42's are going to put it around 2900 @ 70. 3.08's will knock it down to about 2600. (this is assuming a typical 5% converter slip). Tough call. I run 3.73's w/ 28" tire and 3000 converter. Turns 3000 at 65 on the highway.

Id go with the 3.08s .... I just bought a set myself and am figuring on 2596rpm at 70 mph.

plug in your info here to get your cruising rpm: http://www.ringpinion.com/Calc_RPM.aspx

Why not hunt down a set of 3.31s and split the difference?

Why not hunt down a set of 3.31s and split the difference?


Where would I find 3.31's? This is for a 12 bolt also...forgot to mention that.

Jacen,I wouldn't spend two seconds looking for a set of 3.31's. At highway speeds there's less than 100rpm difference between the 3.31's and a set of 3.42's. For what you've described 70-75mph for 3hrs at a time. I would put the 3.08 gears in and go.
Mike

Thanks everyone...that is what I was thinking. 3.08 it is.

My friend and i both bough used 331s form ebay and had them installed yrs back with no issues with the gears & i also found an NOS set of 331's too ,but i unfortunately had issues with the mechanic that installed the the used gears .

The Pinion nut came loose shortly after the install (no loctite was used ) and car is never abused or run at strip which could hlepthe not to come loose under abubse. Now i have slight vibes and slop in pinion shaft even after re-trq of pinion nut to 125lbs with loctite. They also dident put any sealer on pinion spline so gear oil wiked up & out of the pinion splines leaking all over the place too,frgging sloppy ass work IMHO.

That's why i normally do all my own work but this 1 time i said let a supposed pro do it and ended up with issues and thats with a shop/guy thats been in the buisness for over 30 yrs with good rep which i didnt see at all,thats for sure. Now i have to do ring & pinion the setup all over again./GGGRRR!!!!!!!!!

Scott

out of curiosity, why does everyone try to stick to the 26" Diameter. Am I wrong for going almost 29" in the back?? The way I calculated it, the taller tire would make for less RPM's on the highway.

Unless you like the look of a 29" tall tire, there's no reason to run one that tall on the street.
Mike

3.31's - Toms carries US gear, dont know about inventory or those gears but shows them on their website.

Unless you like the look of a 29" tall tire, there's no reason to run one that tall on the street.
Mike


Well I will say this. I was just looking at the 70 (primered) Chevelle they used for the Fast and Furious movie and it has Coy C-55's on it with 17x8 (235/50r17) up front and 17x9.5 (275/55r17) in the back and I think it looks very nice. Kind of a nostalgic muscle car look.

On another note.....I just ran some gear to tire size calculators and my suspicion was right....the taller the tire, the lower the RPM on the street. HHHHmmmmm......decisions.

I have a mild 461 with TH400, ~ 2000 stall, and 2.73's. With the 27" tires, it brings the rear end down to about 2.60.

It's a daily driver and I love the 2.73's. I take frequent long road trips and at 60 mph the car gets about 15 mpg. For every 5 mph about that the mileage drops by about 1 mpg.

The engine is built for low-rpm torque. It will fry the tires from a standstill yet if you ease the pedal you can take off very fast without much wheel spin.

Make sure the downshift on your tranny is properly adjusted. I used to have an Olds with a 455 and 3.55's in back. With the torque and deeper gears, it didn't have and didn't need a downshift cable. But it was a real bitch to drive on the highway over 55 mph. While you may lose some acceleration with highway gears, you'll still be able to downshift from 3rd to 2nd at about 70 mph. That's a huge benefit. For daily driving, a well-tuned downshift cable will compensate for a lack of rear-end gear.

On another note.....I just ran some gear to tire size calculators and my suspicion was right....the taller the tire, the lower the RPM on the street. HHHHmmmmm......decisions.
You're a sharp cookie Jacen,we'll be expecting big things from you.
Mike

I have 3.08s(10bolt) or 3.07s(12bolt) in all my cars, except the 455 Cutlass, it has a 2.41.
With a healthy engine, there's more than enough torque (or SHOULD be) to turn this kind of gear ratio. PLUS, with today's gas prices, hi-geared rears are the way to go.
Ya, they're not that much fun in town (but with a 4sp, just shift into a lower gear), but on the Interstate it's sure nice to keep the rpms at a respectable level. :thumbsup:

Ya, they're not that much fun in town (but with a 4sp, just shift into a lower gear), but on the Interstate it's sure nice to keep the rpms at a respectable level. :thumbsup:

when you say they are not fun in town, what do you mean by that? I am running a a T350 so downshifting is really not an option...lol

I always thought a 700 r4 was a great idea. Hot 1st gear, great overdrive... I know that wasn't the question, but just my .02

--Rich

when you say they are not fun in town, what do you mean by that? I am running a a T350 so downshifting is really not an option...lol

Downshifting should be an option with a TH350. Make sure it's connected and set up properly. When I bought my car it had no downshift. It was a mild BB in a heavy car with highway gears. It wasn't quite a dog, but setting up the downshift cable really woke the car up. That's what gears are for. If you have a 575 that puts out 700 lbs of torque then a downshift cable would do nothing but frustrate. For a mild street setup, use everything available.

I don't think any aftermarket companies make a new 2.73 12 bolt gearset, but it sure was the best thing I did for my '68. I got a complete 12 bolt limited slip 2.73 out of a Monte Carlo to replace my 4.10 rear. If you have the torque, gear it tall as you can. There is a reason why high-revving small blocks need 4.33 or shorter gears - even a big lazy big block will move with 3.08s or 2.73s. With 275/60 tires on a 15 inch wheel and 2.73s I am turning about 2500 rpm at 70, and will still break traction when I downshift to 2nd.