[455GP]

i'm getting 3000 rpm at 80 what rear end do you guys think i have? Also what rear end would help me get a better start off the line, Thanks, Joe

[b-man]

How tall is the rear tire?

Auto or manual trans?

Without knowing at least the tire height it's hard to get close.

[455GP]

sorry it has 14 inch wheels and it's an auto

[b-man]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by 455GP:
sorry it has 14 inch wheels and it's an auto </div></BLOCKQUOTE>
Get out your tape measure.

Measure from the ground to the top of the rear tire.

[455GP]

I've got the car at the tranny shop. Getting a total rebuild and shift kit installed, it should be done tomorrow. I'll measure it then. thanks for the help, Joe

[goatless]

I'm gonna go out on a limb and say 2.93.
But really we do need the tire hieght.
What is the size of the tires? 225/70/14 or...??

As far as gears to get off the line better we need more info. Motor, cam, weight, intended use of the car. Any gears that are lower (higher numericaly) will get you moving quicker but will give you worse milage and could be more annoying on the highway. The trick is to find the right compromise for you.

[b-man]

Joe,
Here's a link to a tech article that I wrote for another board that I visit. It tells you how to find out your gear ratio, the tire size won't matter.

Click here.

But after you measure your tire we can help you better.

[455GP]

25" 5/8th's from the ground to the top of the rear wheel. P225 70R 14. Firestone Indy 500 tires.

Which rear end would you guys suggest? 3.23, 3.42, 3.70?

[b-man]

A 225/70-14 tire should be 26.4" tall, it will measure a little shorter (your 25-5/8") with the weight of the car sitting on it depending on tire pressure.

With a 2.78 gear and a manual trans you'd see 2830 RPMs at 80. Add about 5 or 6% more RPM to make up for converter slippage on your automatic trans and it comes out very close to 3000 RPMs at 80.

Keep in mind that your tach and speedo may also have some error, none are ever 'right on the money'.

To be positive, read the article in the link and check it yourself. I say 2.78s at this point.

For better accelleration use a 3.23 gear. Still good on the highway, but definitely quicker off the line. That's the best I can recommend since you have not posted any info about your engine or the intended use of the car.

[455GP]

72 grand prix 455. I freshened up the top with a 2800 cam and lifter set. pushrods, new timing set.

I just got the car back from the tranny shop total rebuild, including cheetah shift kit.
I'm just driving it on the street.
It's a little sluggish off the line, I'd like to get a little better jump off the line, but like you said, Still good on the highway.Thanks Joe

[b-man]

Since you're running that very mild cam a gear change to anything deeper than 3.23s won't do you any good. Probably the optimum performance gear for you would be a 3.08.

I have a near-stock 455 with the 068 cam (similar to the 2801 cam) in a slightly lighter car than yours, it had a set of 2.56 gears in it. I swapped them out for 2.93s and the car didn't run any quicker in the 1/4 mile.

I'd be happy to have my 2.56 gears back. I think 2.78s are a good gear for your combo for all-around driving enjoyment, but if some 3.08s show up for a good price grab 'em up.

[455GP]

Thanks for the replies a really appreciate it. Joe

[SGTGTO]

AH just get rid of that puny auto trans and go to a Tremec! My 3.55s are like 4.10s off the line, and I cruse at 70 doing 1950 RPM. I love it!