1998 T/A Speedo Calibration

[98droptopt/a]

Help me out. 1998 T/A, LS-1 with an automatic. I am having a problem with the speedo and odometer running about 10% fast at 60mph. It does have the stock sized 245's on it now. Not sure if the rear end gear was changed (had it for 2 days now.it replaced a 95 LT-1.new animal...) but even if it was I need to get this back where it should be. The car drives find. It's not a real tall gear if it was changed. 3 Dealers said it could not be fixed.1 dealer thought it was a wheel speed sensor. Speedo shop said dealer can flash with the tech1 and reset it. Hypertech said it could be fixed if the stock program was in the car. Since the speedo was off I'm not sure it is. The guy I bought the car from said he does'nt know. Is this truely a non fixable deal?

[ConElite]

Hey buddy, welcome to the forum. Lets start this thing.
Everything is fixable. Im sure you can have a custom programming done to your PCM. The main thing you need to find out is what gearing you have. Im guessing the previous owner did some work to your car. You have any idea what has been done to your car?

[Injuneer]

Look at the build code tag in the glove box. You should find the code "GU2" (2.73 gears) or "GU5" (3.23) on the tag. That will tell you what gear your car left the factory with, and what the PCM is probably programmed for.

Then try and determine your current gear ratio. There are several ways:

-pull the cover off the differential and try and count the teeth on the ring and pinion. Divide ring teeth by pinion teeth.

-put it on jackstands and count how many times the driveshaft rotates for one rotation of the rear wheels.

-get your exact tire size (245/50-16's are 25.65"). Drive the car at a fixed "indicated" speed over a 5 mile distance, using the mile markers on a local Interstate. Use a stop watch to get the time it takes to cover that distance. Calculate your actual MPH from distance and time. Get a number of readings of the engine RPM vs. MPH in O/D with the converter locked, and use an online gear calculator to calculate the rear axle ratio, using the "corrected" MPH you calculated. Remember the stock tach typically reads 100-200RPM higher than actual engine RPM. The speedos are usually fairly accurate - assuming the correct tire diameter and gear ratio are programmed in.

Once you get "GU" code off the build code tag, you can look at various conversion rear axle ratios to see which one would cause a 10% speedo error. Not exact, but may point you in the right direction.

Once you have the correct gear ratio, you can us a handheld programmer, get a mail-order PCM tune, or use a software based tuning program to correct the speedo.

[98droptopt/a]

RE: Speedo...

Thanks for the response guys... The car came with a GU2 / 2.73 gear. I have checked the speedo and odometer against my Garmin gps unit. I know just how far it is off. The guy I got the car from just claimed the speedo was off. I figured no big deal... everthing is fixable. Like I said the car drives fine with whatever gear is in it. No exessive rpm or bad tranny shift points. I if I figure out the gear I just need to know a program to calibrate to it. Thanks Again Guys...

[Wild Willy]

Double-check the production tag to see what axle AND what tire size the car was built with. I would have thought a T/A would have come with 275/40-17's, but don't know.
The 10% is an odd amount to be off- I changed out the 2.73 axle in my Z as soon as I got it (went to 3.42's, a 25% difference) A swap to 3.23's fairly common, would be about 18% off.....? I don't even know if they made 3.08's for the F-body, but that would be a 12% difference, close, but why bother making such a small change????
The PCM calculates mph by measuring driveshaft rotation speed, with a look-up table that is programmed with final drive ratio and tire size- It can easily be re-programmed. You need to find the correct values to feed into it

[4THGEN Z]

Double-check the production tag to see what axle AND what tire size the car was built with. I would have thought a T/A would have come with 275/40-17's, but don't know.

275/40/17 and 245/50/16's have the same outside diameter.

[98droptopt/a]

RE: Speedo

[/IMG]Thanks again guys for all the help. I think I should have looked for the obvious first. First of all I had a typo on the first post. It is approx 20% off at 60mph not 10. Garbage in garbage out they say. With 3.23's and a tire that's too small that should put me right in the ballpark of 20% + /-. I will try the program for the 3.23's and I have a set of 255-17's for the car that are 26.5" tall. I was looking for a computer problem.... Previous guy changes gear...Takes program and tires with him = 20%. I'll keep ya posted.

[ConElite]

Very nice. Those look the WS6 rims. Are they 16's or 17's?

[98droptopt/a]

re:

Thanks... standard chrome rims 16". They acually went with my old 95 t/a. That guy wanted them and I really want to go to a 17" rim. It will get polished 17" Torque Thrusts when the weather breaks. Right now it's away in the garage till the snow goes away.

[Injuneer]

..... I would have thought a T/A would have come with 275/40-17's, but don't know....

The stock tire on all the V8 cars if a 235/55-16. If the car included the Z-rated tire option, it had 245/50-16's. The only cars that left the factory with the 275/40-17's are the WS6 and SS models, and the SLP Firehawk conversions.

[98droptopt/a]

Anyone running a 255-50R17? I have a brand new set sitting in my shop. Is that a size to big to be practical?

[Wild Willy]

The stock tire on all the V8 cars if a 235/55-16. If the car included the Z-rated tire option, it had 245/50-16's. The only cars that left the factory with the 275/40-17's are the WS6 and SS models, and the SLP Firehawk conversions.

Tell me about it! That's what my Z came with (as well as the fricken 2.73 gears!?) Leave it to GM to screw up a "performance" car. At least I got the 150 MPH speedo, AND the steel driveshaft. Half the fun of these cars is fixing what GM screwed up. When I got my 3.42 axle, off a wrecked WS6, I took the sway bars, the torque arm, rear springs and the driveshaft. Even the rear shocks, I wasn't prepared to do the fronts- they were DeCarbons, but much larger diameter.

Holy thread hijack, batman! Sounds like you're on your way to fixing the problem- Let us know what you find

[Injuneer]

Anyone running a 255-50R17? I have a brand new set sitting in my shop. Is that a size to big to be practical?

A 255/50-17 is a 27" tire. It'll give you a 5% speedo/odo error, raise the body 3/4" further off the ground, and make it feel like you swapped in 5% numerically lower rear gears (e.g. 3.42 -> 3.23)

[98droptopt/a]

Thanks Guys Fixed and Done

I just want to say thanks for the help. The car did have 3.23's installed. I reset the gear with a Hypertech and it's dead on now....