Post Rear-Swap. What Gears do I have?

[Jersey Mike]

So I originally had a steel driveshaft and what I believe were 2.73s. I swapped rears not too long ago. The seller said it had 3.23s. I took it to TTP & had Matt recalibrate the speedo for 3.23s, however, the speedo is still off. So what gears could it be & how can I find out?


Speedo will show 40, Nav will show 45
Speedo will show 65, Nav will be showing 80.

I had a police officer use his radar gun to check my speed for me, and sure enough I was going faster than the speedometer showed.



So, what gear options are out there for these rears, and what gears could it be?

[sweetbmxrider]

2.73s if you are reading slower than you are going would be my guess

3.23's and 3.42s were the other factory gears.

just mark your drive shaft, turn the wheel one full rotation, and count the amount of times the mark goes by. 2 and 3/4 is 2.73s

[LTb1ow]

3.42s would teh other common stock gear.

[sweetbmxrider]

but if he had a taller gear, his speedo would read faster than his actual speed. like he would be going 50 and his speedo would read 70

[WildBillyT]

Throw a GPS in the car. You can calibrate it for the gears and tires.

[NJ Torque]

Could be the cluster itself is off. Mine is.

[r0nin89]

Yep speedo being off is in proportion not a set number so you definitely have different gears than you were told. Tires can also throw it off but from a truck perspective going for 29in tires to 33s on my old toyota didnt even throw the speed off that much at 65.

Sounds to me like you swapped the speedo gears for 3.23s and got another 2.73 rear end.

My condolences

[Jersey Mike]

Originally Posted by r0nin89

Yep speedo being off is in proportion not a set number so you definitely have different gears than you were told. Tires can also throw it off but from a truck perspective going for 29in tires to 33s on my old toyota didnt even throw the speed off that much at 65.

Sounds to me like you swapped the speedo gears for 3.23s and got another 2.73 rear end.

My condolences

That blows, I hope that's not the case. I bought them off a member here

[sweetbmxrider]

either pull the cover and count the teeth/look for the tooth count marking or spin the wheel and count driveshaft revolutions. make sure your car is on jack stands with wheel chocks and a hard helmet and reflective clothing and scuba tank and oven mitt and riot shield. never know what will happen down there....

[NjbadSS]

I am having this same issue with my z28 right now..I used my programmer to try and fix the speedo and still I am on the highway cruising past cars and look down and see 60ish. Around town I am keeping up with traffic and look at it and it says like 35..40..let me know how you find out and that would help me out, I prefer the easiest method rather than taking the cover off.

[Jersey Mike]

Short of hopping on the dyno & letting someone else figure it out, the easiest method would be spinning the wheels to look at the DS. Unfortunately, I'm at school now & no where near a jack & stands.

[BonzoHansen]

So you guys buy used rears and don't even open the cover to look or even change the fluid?

[LTb1ow]

Originally Posted by BonzoHansen

So you guys buy used rears and don't even open the cover to look or even change the fluid?

Its what all the cool kids are doing these days.

[Jersey Mike]

Originally Posted by BonzoHansen

So you guys buy used rears and don't even open the cover to look or even change the fluid?

Checked the fluid, it was full, just like the seller said.
And what can I say, I was a dummy for listening to the person I guess.

[NJ Torque]

Bring it by one day we'll check it out... Can do it by just driving.

[Jersey Mike]

Gimme an address and I'll be there.

[98tadriver]

to be 100% sure of what gears it has, pull your rear diff cover off and count the teeth on the ring and pinion. then divide the big number into the small number

another way to do it is by jacking the rear of the car up and turning the tire 1 whole rotation and counting how many times your DS spins, but sometimes, rears are beat and it wont be accurate

another way is to drive it and see what rpm its at, but theres a chance that the good ol gm tach is off too. just pull the cover off. it wont take long

[sweetbmxrider]

Originally Posted by njbadaSS

I am having this same issue with my z28 right now..I used my programmer to try and fix the speedo and still I am on the highway cruising past cars and look down and see 60ish. Around town I am keeping up with traffic and look at it and it says like 35..40..let me know how you find out and that would help me out, I prefer the easiest method rather than taking the cover off.

could be a combo of tire size and gears

[Tru2Chevy]

Tell us what size rear tire you are using.

If you are on stock 16s, it should be a 245/50/16
If you are on stock 17s, it should be a 275/40/17

And shame on you for not at least popping off the cover and changing the fluid. I don't care if I bought a rear from a blood relative and they told me that there was brand new synthetic fluid in there, first thing that would happen is pull the cover, inspect everything, and then fill it up at home.

- Justin

[Jersey Mike]

Originally Posted by Tru2Chevy

Tell us what size rear tire you are using.

If you are on stock 16s, it should be a 245/50/16
If you are on stock 17s, it should be a 275/40/17

And shame on you for not at least popping off the cover and changing the fluid. I don't care if I bought a rear from a blood relative and they told me that there was brand new synthetic fluid in there, first thing that would happen is pull the cover, inspect everything, and then fill it up at home.

- Justin

They're the stock 16s. Can't remember the tire size off the top of my head but I'm pretty sure it's 245/50.

[Czop418]

Originally Posted by r0nin89

Sounds to me like you swapped the speedo gears for 3.23s and got another 2.73 rear end.

Nope if he bought a rear with 2.73's and then recalibrated for 3.23 his speedo would be reading faster than what hes actually driving. Sound to me like there 3.42's.


Heres the year and gears. for 4th gens

2.73 A4 93-97
3.42 M6 93-97

3.23 A4 98-02
3.42 M6 98-02

[r0nin89]

No thats wrong. When ever you de-gear something your speedo will show you slower than what your driving which is whats happening. It doesnt matter if you go to higher gears or bigger tires it does the same thing.

If his speedo is slow and he has stock size tires then he has a higher ratio rear than hes speedo is calibrated for. Seeing as how its calibrated for 3.23s if he had 3.42s hes speedo would read faster.

[Czop418]

Ha wow i just read what i posted. Yea if you had bigger gears you would be reading faster not slower. My bad. I had to think that out for a moment. But yea they're 2.73's sorry bud.

[S.J.SLEEPER]

Originally Posted by BonzoHansen

So you guys buy used rears and don't even open the cover to look or even change the fluid?

yeah, that was what i thought exactly.

pull the cover off and count the teeth on pinion and ring

ring/pinion = ratio

maybe you have 308's in it

[creeper]

the guy selling the rear said he wasn't 100% sure, but because it came out of an automatic car with an aluminum driveshaft and RPO code GU5, it would make sense for it to be a 3.23, since to his knowledge it was the original rear in the car.

also, before anyone questions the integrity of the seller, you need to understand the scenario. the guy had no plans of selling the rear at all, he had actually just gotten the car back on the road. when i called him and said a friend of mine needed one for his car, he was kind enough to drop it for me the next day since he has another vehicle he can drive for the time being.

he wasn't trying to trick anyone, mike needed a rear(and told me he would still take it if it were 2.73's anyway) and this guy had one. had it been 3.23's it would've been a plus, if not at least he has a car that can move.

Originally Posted by Jersey Mike

They're the stock 16s. Can't remember the tire size off the top of my head but I'm pretty sure it's 245/50.

yep they're 245/50's