I was told my Z-28 had 3.73's when I bought it a couple years ago. I'm curious to know if it actually does or not because I'm installing headers soon and am getting my PCM tuned for my mods. So, how do I go about knowing whether or not I have the gears or not?

The only thing I can tell you is that the speedo is off by a pretty good margin. I have 275/40/17 inch tires, I don't know how close those are to factory height...so I guess that might be a factor as well.

 

I remember 3.73s not coming stock.. but check out your build numbers and post 'em up and I'm sure we can lead you in the right direction.

 

If your speedo is off, there won't be a way to tell by that method. 3.73s did not come as stock. You can jack up the rear of the car and count how many times the driveshaft rotates for one revolution of the rear tires. 3-3/4 turns on the driveshaft would be about 3.73.
Another way would be to remove the rear end inspection plate and read the stampings or count the teeth.

 

275/40/17 are the factory SS size, but still the same as 245/50/16 (also factory size). If the speedo is wrong it's likely due to the gear change, but whoever did it, DIDN'T re-cal the speedo .

BTW, A4 or M6? (curious what ratio you would have had stock ..... you can check the codes on the driver's door jamb, or maybe in the glovebox, for either GU2 (2.73's), GU5 (3.23's) or GU6 (3.42's)).

Depending what gears your car "had" would help tell if you do/don't have 3.73's by how far off the speedo is.

However, Shoebox's methods for finding out what gears you have are the only for sure ways to find out .

 

It's an M-6. I know the car didn't come with 3.73's stock, he told me that the car had 3.73's in it though. He was the second owner of the car and told me the first owner had them put in. I know it's kind of a ridiculous question but I just wanted to be sure before I had the programming done. I'm not sure if gears make a difference in programming, but if it does I want to know. I guess I'll be raising the car up this weekend and finding out for certain.

As far as looking at the codes, I'd have to check but I think the driver's door was repainted and the code sticker is missing. The car isn't perfect by any means, but I'm still happy with it.

 

I don't know the exact formula, but if you jack the rear up and spin the tires a complete turn, the driveshaft should spin around 3.7 times if you have 3.73s. This is coming from Jeeps, though, so don't hold me to that.

 

Reprogramming will correct your speedometer but of course you have to provide the correct gear ratio to your tuner.

 

Read post #3.

 

Yeah, if you actually type out the ratio instead of just the output number, then it makes more sense. 3.73:1. It literally means the output it 3.73 times greater than the input. But you knew that already, didn't you?

Knowing that already, I don't see why it never occurred to me just to lift the rear up and spin it around.

 

Let your tuner know the gears and tire dia. He'll be able to fiw the speedo.
For fun you can go here: http://www.f-body.org/gears/

 

Actually, it means the input to the rear (driveshaft) rotates 3.73 time for each rotation of the output (axles).

 

Yeah, sorry shoebox.. didn't mean to steal your thunda. For some reason I was thinking (Early this morning) that I was the only one posting in here.. didn't even think to check.

 

Also, with a 26" tall tire in 4th gear you should be going about 45 mph about 2000 rpm.
Or, the driveshaft should turn 3.7 times for one tire rotation.

 

Well, let's put it this way ... you said your speedo was off by a big margin? If your car has 3.73's, but the computer was never told, then at 60 mph, the speedo would in fact read ~65 mph:

60 x 3.73 / 3.42 = 65.4 mph

And again, your tires are the correct size (diameter), so they shouldn't be making any difference.

Oh, and knowing that your car is an M6, you don't need to bother checking the gear codes ... we know it would have come with 3.42's (GU6) from the factory. The only reason I asked you about that is because if your car was an A4 and originally had 2.73's, but now had 3.73's, the speedo would be WAY off!!

60 x 3.73 / 2.73 = 82 mph!!