Hi

I am trying to recalibrate a 95 Z28 M6, which has gone to 275/40/17 tires and a 3.73 gear.

Tunercat at first listed the car as an auto "Y or B car", with a 3.07 gear and 285/40/17 tires... but I think that's just a Tunercat default.

So I corrected everything, F car, manual, 3.73 gear and 275 tires. I hit calculate, then apply, then ok. Then to be sure, I read the PCM and saw that my changes had indeed been uploaded to the car. The speedometer is still wrong though, it's about 10% high.

Does anyone know if there's a writeup anywhere that explains how to do this? In my searches, I have read that you have to adjust the values listed (speedometer scalar and all that) to certain levels to get it to work. Maybe someone with the same setup can let me know what their values are?

Thanks for your help.

 

I used the calibration tool and changed tire sizes untill my speedometer was accurate. It took about 3 tries untill I got it right. I think I ended up making my tire size in the tool larger than what I was using on the car. You could also use the revolution per mile option instead of tire size or play with the rear ratio. After you change something if your speedometer is closer to correct you know you are moving in the right direction. Mine was also off by about 10% using the tire size that was on the car.

 

I believe tunercat has a mess up in their formula because if you do have it calculate the speedometer for you it always ends up being considerably off. When i had my 3.90 gears in my car i used tunercat and my speedometer was off by about 5mph slow at 75 if i remember correctly. Meaning when my speedo said i was running 75 i was actually doing 80. I just took the 75/80=.9375 to get my correction factor. I then took that by the gear ratio i was actually running which was 3.90 and that gave me the gear ratio i needed to put into the pcm. So my calculation was .9375 x 3.90= 3.65625. After i changed my gear ratio to that my speedo was spot on and i tested this with a GPS. I hope this helps.

 

Just had the same deal on an Impala the other day. It was off by about 5 mph at about 70 mph. Just figured the percent difference and then changed the gear ratio by the same percentage. Checked it with a GPS and it was withing .5 mph.

Todd

 

I've recalibrated mine for a couple of tire sizes and a gear change and had no problems. This is on a vette with AT, so if there is an error maybe it is in the F-body or MT calculation.

 

nope, I have an automatic and am in the same boat. Currently it is calibrated for my 18" vette wheels, but I am running the stock 16" wheels this week for the track. I used the Tuner Cat recalibration tool and the speedo does not work at all. So I put it back to the 18" calibration and now it shifts funny at WOT and high RPM's.

 

Thanks for the replies.

I decided I'd just compensate by putting in 4.10 as the rear end ratio, which I did last night, but it still didn't change anything. Every time I read the PCM it still has the 3.73 gear listed. I think I am doing something wrong.

I looked through the Tunercat manual but there's only a small part on the speedometer recalibration and it doesn't go in to any detail (which lead me to believe it was relatively simple). I am putting in the numbers, pressing calculate, then apply, then ok. Is there something else I need to do?

 

You aren't reading 3.73 from the PCM. No such setting. That's just in tunercat.

My tunercat remembers the last entries I made in that dialog, so if yours comes back as 3.73 after you put in 4.1 before then you may have a problem.

Just to make sure we're not missing the obvious here - are you actually programming the PCM? i.e., you choose "Tools/Program PCM" and then a dialog comes up and 5 min later it finishes programming?