97 Y87 powertrain question

[elc32955]

I've been trying to figure out what gear ratio is in my 97 Firebird for a while now. The car has the Y87 package w/factory Posi and was originally delivered with the GU6 (3.42) gears, however the first owner of the car had the diff. changed and the speedo is off. I've done the T/A cluster swap and I'd like to get the speedo reading correctly if I can. When I'm doing 70 on the highway (verified by a radar clock) the speedo is reading 74 MPH. For the life of me I can't remember if that means the gear ratio is greater or lesser at the driveline (I.E. do I have 3.23's or 3.73's)?? I've got 3.73's in my 9C1 Caprice and that was one of the best mods I made in the car (although it killed the fuel economy).

I don't think there's a stock calibration for a '97 Y87 with 3.73's, however if it has 3.23's can I just reprogram the PCM with a 3.23 VIN and correct everything up with a stock calibration? Which gears do y'all think I have based on the speedo error?

Thanks!
Eric

[andrew.brandon]

Isn't all 3.23's 3.4L gears with OBD I? Would that work with a 3.8 with OBD II?

[elc32955]

Yeah, that's the other problem, programming the PCM. I can program with Tunercat for OBD-1 for the B-Body's but I know OBD-II solutions are still pretty scarce, that's why I was wondering on a factory calibration from the friendly dealer... [img]smile.gif[/img]

Thanks
Eric

[Dan Vincent]

A speedometer that goes "faster" than your actual speed means you have shorter gears... you can counter this either by reprogramming or changing your tire sizes (IE put on 17 inch rims with taller tires).

Best way to know this for sure is to pull off your rear end cover and count the gears on the ring and pinion.

[mjparme]

Hmm, I can't get the math to work out to figure out what gears you have.

If the previous owner went from 3.42->3.23 then the speedo would show you going slower than you are actually going not faster. However, a 4 mph doesn't work out if it was 3.42->3.73.

1 - (3.42/3.73) = .0832

70 + (70 * .0832) = 75.824

[Dan Vincent]

If he's running tires other than stock, that could throw it off too.