I have a 91 RS. Supposed to have 2.73 gears. Calculator says at 70 mph with 13inch radius tires, 0.70 4th gear, and the 2.73 I should be at 1728.7 RPM. HOWEVER, in reality, I'm at around 2400 rpm at 70mph. The calculator says at that RPM with that tire radius and trans. gear, its more fitting of 3.73 gears (2361 RPM).
The trans shifts fine, it has slipped once (on a 4 to 2 shift) in the past nine months, but has shown no other problems, with 108,000 on the odo. Also, it does shift through all four gears. Is there a problem with my trans or is it at all possible to have 3.73s.

 

I want to add that, according to title changes on Carfax, I am the fifth or sixth owner. No other apparent mods, but the car is very well maintained.

 

If your rearend is stock and untouched, you definitely have 2.73's. However, if anyone did change the gears before you owned it, the only way to know for sure is to pull the cover and find the ratio that's stamped on the back of the pinion gear or the side of the ring gear. There will be two sets of numbers with a colon between them (ex. 3.73 is 11:41). You just divide 41 by 11 to get the ratio.

 

DAAAAMNNN. I wish I'd have known that two days ago, I just changed the diff. oil.

Nex time

 

Converter lock up working?

 

First thing is, what makes you think your engine is at 2400 RPM while driving down the road?

Is this the number that waving-around thing the factory put in your dash is covering up? If so, it's probably wrong; like most all the rest of them. THey're not "instruments", they're just interior trim, to help augment the sporty image. After all, no car can truly be a "sports" car without one of those, right? Doesn't matter if it means anything or not, it's just gotta wave around when you give the car gas. Then it's all good, and the car is a "sports" car. Ask any Honduh driver.

Before getting all worked up and excited about gears, check your engine RPM with a piece of REAL test equipment. The most likely explanation is that your gears are stock (since they cost LOTS of money to change) and your tach is broke (since the tach breaking is free). Knowing typical car owners, I know which one I'd bet on.

 

Sofakingdom has pointed out the most probable cause but you may want to consider the most dangerous one as well. No matter how long you run with a busted tach it will never result in a $1000 transmission rebuild.

I do like that guy though.

 

Hahahaha.
I'll admit I'm being too optomistic about this.
Also, I exagerated, its 2200 rpm at 70 mph (according to that wavy thingy ).

What is the "REAL" test equipment you're talking about? Are you talking about an aftermarket tach?

I thought it was. How can I test it?

 

Ignition / tune-up analyzer, at a shop.

THey shouldn't charge you more than a few minutes labor to just hook up to your car, and give you the real engine RPMs vs your tach readings; i.e. hold the engine RPM to where the dash tach says 1000 RPM, give you the real RPM; hold the engine RPM at 2000, give you the real RPM; etc. Up to 4000 or so should be enough.

The way these tachs fail is, they read proprtionally higher. As they age, they read higher and higher. Say, it turns out that your engine is actually doing 800 RPM when the factory trim package reads 1000; and 1600 when it reads 2000; and 2400 when it reads 3000. That's 20% at all RPMs. It's not like they're "500 RPM off" like some people say, which would be sort of like having "zero" offset from zero.

I've seen these as far as 100% off, and still "appear", to the eye, to be working normally; i.e. reads 2000 when the real RPM is 1000, reads 4000 when the real RPM is 2000, etc.

 

well my car runs 3000 on the dot at 70 and it has a 2 month old Equus Tach. is that what it should be running?

 

https://www.thirdgen.org/newdesign/t...ulations.shtml

 

for a sixer wih 3.42s, 3000@70 seems pretty close

 

how about you place a jack under the differential housing and jack the car up, place jackstands under the LCA axle mounts. put the car into neutral and count how many times the driveshaft turns to make the tire do one complete revolution. that should give you a good idea of the gear ratio. i did this on my car and got almost 2 3/4 turns which means i have 3.73's (and ive verified that by looking at the ring gear)

 

Check your lockup by tapping on the brakes while keeping the throttle at highway speed cruise or gentle acceleration. Brake switch will disengage lockup and cause an rpm flare of about 200-300. If you've been cruising without lockup your converter slip allows the engine to turn at a higher rpm than needed...>0.70 through trans to rearend.

 

I'll try that. But how does 2 3/4 equate to 3.73 gears?

 

i meant 3 3/4 turns, sorry

 

Hahha, its cool. Wow, i was freakin myself out trying to figure out how 2 3/4 led to 3.73.

 

Yeah, and 2 3/4 turns would be exactly what you'll end up with for the 2.73 ratio!

 

I just got my ALDL cable and I thought I'd update.

I do have 2.73s, and my actual RPMs are MUCH lower than what my tach is telling me.
At idle, tach reads about 800, actual is 650.
At 4,000 rpms on tach, the engine is actually just breaking 3000 rpms.
It is off by 1000 rpms. That is ridiculous.

 

Now that Sofa guy is going to get a big head...

 

hahahaha dang this thread is old

did u ever try the driveshaft spinning thing?

 

Yep. That is what I did to find out I have 2.73.s I also did it on my friends 86 v6 camaro, and it has 3.42s, which is what it should have.

I just brought this thread back to life to let people know how misleading the stock tach is.

 

Yep. I found that out on my old '91 GTA. I always wondered why the motor would completely quit pulling at 5500 RPM, until I hooked up a datalogger and it told me when I was really shifting at 6600RPM even though the tach said 5500RPM. I'm surprised that 160k mile TPI motor held up as well as it did

 

Wow...13" radius tires...12" wheels? That'd be some super low profile tires!

 

Sometimes there's a difference between radius and diameter..

 

i redid the TV cable adjust ment and my RPMs dropped from 3000 to 2500 at 70! holy crap im surprised that my trans hadnt burnt itself out since i had put 5000 miles on it that way. Gas mileage went up a lot to.