I've read before that a 3.55 rear end was available from GM at some point... I was wondering if it has ever been offered on a Thirdgen stock.

Also, how accurate are the tachometers in our cars?

The reason I bring this up is that I took my car out to the "strip" for some high speed runs and by my calculations (RPM vs speedometer at speed) I am running at speeds slower per RPM than the calculator on this site. So I punched in some rear end combos and the gearing that fits (perfectly) is that of a 3.55 rear.

My RPO's say 3.23 rear and I didn't think it had been changed, but my tires are VERY close to the stock ride hight and according to the roadside radar detectors, my speedo is within 1 MPH of actual speed.

So is it more likely that I have a tach that is about 10% off or that I have a 3.55 Rear end?

 

AFAIK there is no 3.55 gear available for either iof the rear ends that come in these cars. More likely you have 3.23s. or 3.42 if it's a 10-bolt, or 3.27s or 3.45s if it's a 9-bolt.

The tach is a trim option, not an instrument, even though it sort of resembles an instrument in some ways. Check it against a real one at a tune-up shop or something.

 

the factory 10-bolts with a borg warner t-5 manual transmission were offered with a 3.08 gear, all 5-speed manuals came with these gears, no optional gearing.

 

89maroRST5, I beg to differ.

I have an '83 Trans Am, original T5 and my rear gear from the factory is open 3.73. Posi's with this gear was available this year also.

The Chilton's for 3rd gens has all the codes, gear ratios and posi/open availability. Don't have it with me at present otherwise I'd tell you for sure all the available gears. I think at one time, 3.55 WAS an optional gear, but not for sure.

If this info is vitally important, and no one else has the manual, I will look it up tomorrow and post.

HTH,
Poncho

 

Cool, thanks for the info... weathers getting warm again, so I think I'll crawl under it this weekend and find out for sure... I would love to find a 3.42 or 3.45... but it's probably a 3.23 like my RPO says... I think maybe I should get a better tach.

 

I've never seen a 3.55 for a 10-bolt OR a 9-bolt. Our '79 vette has 3.55, but that's the only one I've ever seen. Heck, I don't think even the 9" has 3.55 as a commonly available option.

 

I kow they make them but I have never seen one in a third gen. My dad's 66 vette has a 3.55 gear in it thats about all I have ver heard of them

 

3.55 is a common mopar rear end ratio. alot of people run the mopar 3.55 sure-grip rear end