ROTICKA

I can't figure out why my car revs so high at highway speed. The car is an '84 Trans Am 5.0 H.O. The motor is a 350 tpi from an '88 T/A. I have 700R4 tranny, 3.42 rear end, and 215/75 R15 tires. I'm running 2600 rpm in OD at 65, 3000 rpm in OD at 75. All the calculations that I have made say that I should be running around 2400 rpm in OD at 75. Could it be timing? Could it be TC lockup? Any info greatly appreciated.

Sonny
Life, it will kill you down the line.

Apeiron

The factory tach isn't exactly known for its accuracy.

Vader

Whaddya mean? The factory guarantees the accuracy of those fine, precision instruments to a tight tolerance of ±65.00000001% of indicated RPM.

Theoretically, that tire, gear, and a TH700-R4 in OD with a locked TCC should net 1,743 RPM @ 60MPH, or 2,179 RPM @ 75MPH.

ROTICKA

So, then, if the tach is right (I checked the RPM on the highway again yesterday). And at 75 mph I'm taching 2900 to 3000. What would be the reason? The car really feels like it needs to shift. If this helps...At 45 mph in OD I'm taching about 2000 and the car feels like it should have another gear, but I know it's cycling through all four, I can hear it and feel it. Thanks for letting me pick your brains. BTW, Ignition timing wouldn't cause it would it? I can't find the timing mark on the darn thing.

Apeiron

How sure are you that you've got 3.42's?

ROTICKA

Well the RPO codes in the console box are what I'm going off for that. But I guess you're right, they could have been changed. How would I find out for sure?

Apeiron

Pull the diff cover off and count the teeth on the gears.

sofakingdom

iTrader: (1)

How are you coming up with that RPM number?

You aren't thinking somehow that the thing in the tach with the hand that waves around as the engine speeds up and slows down, is some kind of test equipment or something, are you? Because IT'S NOT... it's a trim package, designed to promote the "sporty image" of the car. It is IN NO MANNER WAY SHAPE OR FORM an "instrument", even though they have the nerve to call it that on the build sheet.

When these "tachs" fail, they do so by reading higher than actual RPMs. Sometimes, WAY HIGHER.

I have about 4 or 5 of them; out of them all, the one that reads closest to actual, is in my car of course. It only read about 2 or 3% high when I put it in. They go higher as they age, so who knows whether it's still accurate. IIRC the next closest one reads more than 10% high. The rest read even higher.

Don't get all wound up about gears and revs and all that, until you check the REAL RPMs with a REAL tach. Odds are, you'll find out that your tach is no different from everybody else's.

Incidentally Vader, one of those that I have must be outside of that factory tolerance.... it reads almost exactly 100% higher than actual. Twice the real RPM. I guess I need to save that for somebody that has a TBI motor or something, that wants it to make power at 8000 RPM.

ROTICKA

I'll put a real tach on it and see what happens, you're right, I must be sure my info is accurate if I expect good advice. I'll let you know what I find out.

Apeiron

Save it for a high perf Iron Duke swap.

ROTICKA

Okay guys,

I hooked up a real tach to the car. It read close enough to the factory tach to say the numbers are the same. So using the calculators I could find on the web, I've come to the tentative conclusion that I have 4.11 rear end gears. Until I verify that I think we can call this case closed. My '85 T/A does have 3.08 rear end. So a swap would be helpful, to attain any kind of fuel mileage on the highway right? Thank you for the direction.

Sonny

sofakingdom

iTrader: (1)

The car very likely came with 3.73 gears.

4.10 is possible; those come in some S trucks and are available in the aftermarket.

If you want to know FOR SURE what gear you have, the only way is to pull the cover.

If you haven't changed the fluid lately, you're neglecting the scheduled maintenance. While you catching up on the overdue work you should already have done, make a point of looking at the gears; and identifying the carrier, so you don't have to ask the usual "do I have a posi" question and then wade through a bunch of stupid drivel about 2 stripes and all that.

DNSTA

You don't need to pull the cover. Turn the driveshaft by hand and count the wheel rotations. Simple. Easy to tell the difference between 3.42 and 4.11. Give it a try.