chevy_cowboy

Hi guys, I bought my 89 IROC about 10 months ago, almost immediatly I suspected something wasnt right about the gear ratio. The ol 305 seems to turn alot of RPMs to get going any kind of speed. Well I found this site and the ratio calculators here and decided to find out once and for all.

here's the information I Know... Its a 700R4 tranny, RPO codes say it originally had a 2.73 Posi. It's still a posi but theres no way its still a 2.73

the tires on it are 215/65 R15 with around 12.25" radius

In drive the Tach's showing 3750RPM's @60mph
In overdrive its 2750RPM's @60mph

If I'm understanding the ratio calculator correctly, the rear thats in the car now appears to be a 4.73?

also the car downshifts when it shouldnt be causing the engine to overrev, and downshifts and at low speeds upshifts too soon... at 35mph runnin through town it shifts into OD occasionally, is this because the ECM doesnt know the gear ratio has been changed or is it a completly seperate problem?

Ok, assuming my figures are right and its really a 4.73, I deffinatly want to get rid of it. Not sure I'd want to go back to a 2.73... that seems kinda high, what ratio would you guys recommend that'd still give me decent mileage and okish acceleration with the ability to smoke the tires once in a blue moon?

If any of the above calculations are wrong please correct me... I've been awake all night, mabey my brain isnt workin so well lol.

Thanks
Paul J.

DJP87Z28

No such gear ratio as a 4:73. Stock ratio is 2:73 with the 700R4 and your problem might be your tanny is slipping,

chevy_cowboy

Hmm sorry bout that, I knew there was a 3.73 so just assumed there was a 4.73 too.... looking at moser's choices of ring and pinions it must be a 4.56 or 4.88?

The tranny shifts nice and firm and pulls good through all the gears so I dont think its slipping.

Paul J.

ede

pull the cover and count teeth, or look for the ratio stamped o na gear then you'll be 100% sure of what you have.

Rustydawg

If I were in your position with a mystery rear end gear I would jack the rear of the car up, mark the driveshaft with a dab of whiteout, chalk, whatever - and have someone watch the driveshaft and count the turns while you slowly turn the road wheels one complete revolution. It's easier and cleaner than removing the diff cover and looking for numbers or counting teeth.

The number of turns of the driveshaft are your axle ratio, so expect something like two and three quarters turns to just over four complete turns. I've used this method at the wreckers all the time.

I thought that 4.11s were the shortest gears made for the 7 5/8" diff but maybe there are 4.56s out there. You should have no trouble selling that rear axle if that's what is inside!

RB83L69

Have you changed the rear end fluid since you got it? Then it's about 10 months overdue.

While you're doing the overdue maintenance, make a point of looking at the gears.

People do change gears, so it might have something other than the stock 2.73 or whatever in it. It might also turn out that your tach is just unusually defective. The tach in these cars is notoriously inaccurate, and almost always reads higher and higher as it fails. The factory seems to think of the gauges as a "trim package", not as "precision instrumentation", so they're not the finest indicators in the world.

I would not mess around with all the turns counting or the "codes", that's just too much like work when there's this other thing you should be spending your time on instead.

Dewey316

one last thing, i went throught this same thing, wondering how in the world i was going on, the calc kept telling me 4.56's, well low and behold put her up on the dyno, hook the dyno up to the #1 plug wire, i am watching the Dyno's tach readout, look at my tach, dyno says 4000RPM, my tach is PEGGED! mystery solved, i did have 3.08 gears, and i also have 1 bad tachometer

chevy_cowboy

Well, first of all, thanks alot for all the replys...

I spent a couple hours messin with the car tonight.... I got under it and had someone turn the wheel while i counted driveshaft revolutions... aproximatly 2 and 3/4 turns of the shaft for one revolution of the wheel... hmm thats odd, I break out my old craftsman engine diagnostic tester and hook up the tach.

at Idle the factory tach in my car is around 150rpm's off... at 3500rpm's its more like 3000rpm's off

to make a long story short... RB83L69 and Dewey316 were right, the rear end ratio is still 2.73, but the tach has some serious problems lol Fortunatly I've got a set of Dakota digital gauges sitting in my living room waiting to be installed, so that should solve the problem.

Thanks alot for all your help guys, I owe ya one :hail:

Paul J.