I have a 2000 Camaro SS 5.7L M6. The research I've done shows my car shipped from the factory with 3.42 Gears (GU6) door code. The tires and rims are stock factory sizes. I've owned the car about 8 years and the speedometer has always been about 10% slow. If the speedometer says i'm going 40 i'm actually going 44. If the speedometer says 70 i'm actually going 77 (Tracked with GPS and my wife following behind me in her car). The dealership I purchased the car from said the reason it was traded in was because the gears in the rear differential failed and that they had to rebuild the rear end before reselling the vehicle. I'm not 100% sure but I think they may have used the wrong gear ratio when rebuilding it. What gear ratio would they have used that makes the speedometer 10% slow? 3.23? 3.08? or the other direction like a 4.1? I've seen tools sold online for around $200 that will allow you to change the ratio in the computer to fix the speedometer issue but they're like $200. Seems like a lot of money for a tool I'm going to use one time. I was hoping to find a local shop with the tool and try to get them to do it for $50-$75 but I'd like to go in there with the right information so they don't have to jack the car up and manually check the gear ratio. After 8 years i'm finally ready to make the investment to get the issue resolved. Any information would be appreciated.

 

If the speedo says you're going slower than you actually are, then they stuck a lower (higher numerically) gear in it. Without doing that math, I would suspect 3.73 since the difference isn't much. But only way to be sure is pop the cover and look at what is stamped in the ring or count the turns of the drive shaft vs the wheels.

 

check your rpo codes and see what was in it from the factory. it may have had a 342 and they found a deal and replaced the whole rear assembly with the more common 323. much cheaper/easier than rebuilding the unit. you can get a better idea by truning the rear wheel and counting the times the drive shaft turns. if it turns about 3 and 1/4 times its 323, if it turns about 3 and 1/2 turns its 342,,, you get the idea. you have to make sure both back wheels are turning the same. if its posi and its a good they will. rpo for 323 is gu5 and for 342 its gu6. rpo list is on a sticker, can't remember where, my son has my 2000, his engine blew in his Audi, lol. look in the glove box, center console and door jam on driver side I believe. my 93 is in the glove but I know they changed where they put it sometime.

 

Oh crap. Yeah Crabby you're right...I have it backwards. If he's going faster than what the speedo says, then the gears would higher (lower numerically). So yes, good chance it's a 3.23 rear end.

 

What I've found is most of the automatics had 3.23's (GU5 Code) and the manuals had the 3.42s (GU6 Code). They probably were able to get the more common 3.23 rear end cheaper so I'm sure they just did that (It was kind of a shady mom and pop dealership). I'm sure Chevy used the different rear ends for a reason so the question now is what am I losing by having 3.23 gears in the rear vs 3.42s? Would any difference even be noticeable? I daily drive the car in the city and don't do any racing and very little highway driving. Thoughts?

 

If your concern is that the speedometer is off now because having a 3:23 rear axle, you should be able to correct that at a dealer with a Tech 2. I believe with a Tech 2, it will allow the technician to change the rear axle ratio setting in the PCM so the proper speed is calculated to display on the speedometer. I would call a Chevy dealer to verify if that's possible. If you're concerned about not having the 3:42 gear ratio anymore and it causing driving differences, the cheapest solution would be find an axle with that gear in a salvage yard and change it out.

 

Not much difference between 3.23 and 3.42. 3.23 will give better mileage and the 3.42s give better acceleration with the go pedal, but you probably won't really notice a difference with either. Having the M6 is a big benefit. M6 you can go steep with the gears (4.10, 4.11) and still have good highway manners.

 

I broke down and purchased a Hyper Tech Speedometer Calibrator. I programmed the computer about 7 times before I finally found a setting that worked 99%. I had to set the tire size to 25.75 and the rear end to 3.73 to get the speedometer to be within ~1 MPH according to the GPS of my head unit and phone. I guess MKCoconuts was right in his first post. Cheers!

 

I have 4:10 gears, speedo was callibrated with ODBII after market tuning devise, speedo is bang on