I recently installed 20 inch wheels (9X20 255/35 and 10/20 285/30) on my 2007 GT/CS. I have an Airaid CAI with Bama tune. I updated the tune after installing the new shoes but my speedometer in off. It reads about 10% fast. I have talked to Bama and we adjusted the tire rotation to 785 rev in ECM. Did not help, error stayed the same. Anyone had/have this problem any suggestions? Thanks.

 

from stock tires 65mph to the 285/30 63.76mph. I would have left it. But your saying your speedo is off more than 1.24mph?

 

Yes. I agree, if it had been 1-2 mph I would not worry about it but 10% is huge at 70mph.

 

Wow that's a lot

 

This sounds pretty odd, 10% is really high. We can definitely look further into this for you and see what's going on! Shoot me a PM with your order number included and I'll have my head tuning specialist Mike Wilson contact you directly. He'll be able to get you squared away in just a few minutes!

I hope this helps! Our Bama Performance tuning specialists are here in the building Monday-Friday from 9AM-5:30PM EST and Saturday 9AM-5:30PM EST. You can hit them up directly at 888.226.9764!

-Dan

 

I went from 19" to 20" wheels and my speedo was off enough to bother me, I want it as close as possible. I figured out how to adjust it using my SCT tuner and dialed it to the tire size and it was still off. I used a GPS as well as a speedo app on my phone and both were the same so I just kept playing with the adjustment until I got the car's speedo to match the GPS and speedo app.

 

The wheels size doesn't have anything to do with the speedometer being off it's the tire diameter. The stock tires are 27 inch diameter you need a comparable diameter in the 20 size for the speedometer to be accurate. Anything else and the computer need to be adjusted.


Earl

 

Get the revs/mi off the tire manufacturers web site. It will give the best spec compared to all those internet calculators.

 

Which speedo app? Does it just use GPS to estimate the speed or do you have an OBD II adapter that actually reads the speed from the ECM?

I've noticed my speedometer starts getting ~5 off @ 70mph. I hooked up my SCT tuner to my laptop and did some data logging (you can do the same thing with a blutooth OBD II adapter and an app like Torque) and found out the actual speed the ECM saw almost exactly matched the GPS up to 85 mph (when I had to slow down). Its my speedo gauge itself that actually reads off. A lot of people have come across this.

 

True, I mis-spoke. I tried using the numbers off of Nitto's site and they didn't lead me to an accurate speedo reading. I used several other charts I found online and none of them seemed to work for me, so finally I just started using the GPS and speedo app and dialed it in.

It just uses GPS, but I verified that the GPS and speedo app both read the correct speed in a different vehicle before I started calibrating my speedo. That's going on the assumption the 1st vehicle was calibrated correctly but it was a 2011 Volvo C30 that was less than a year old at the time and everything was stock so I felt reasonably safe that the speedo app and GPS were accurate.

The app I'm currently using is called "InDrive".

 

gm does this with their speedos reads faster than you are actually doing by about 10% so you think you are getting better mileage. my toyota truck is the same way, went from 245/75/16 to 275/70/17 and now my speedo matches my gps which is + or - .5%. probably all mfg. do this,plus you get thru the warranty quicker. i have not checked my gt, changed out the 17's with 285/40/18 nitto's. according to tire rack info just about 1/10 inch differencence than the stock tires.

 

I'd hope that if both devices use the same method to determine the speed, they'd both give you the same result. Problem is, even with testing against the volvo, thats really doing a sanity check on the GPS /and or APP are giving you a good reading. Don't get me wrong, that's an important thing but its not telling the whole story. Whether its the speedometer doesn't indicate accurately or if the ECU's calculation of your speed (based on wheel size and rpm) is off.

If you change the wheel size setting in your tune, you're not just affecting the speedometer, but also your odometer and any calcs that depend on it (like MPG and miles to empty). A cheap blutooth adapter is only $20 and Torque Lite is free. Not only will you know whether its a wheel size problem or a speedometer needle problem, but you'll get access to everything the ECU keeps track of (from coolant temp to engine load, etc). Just something to keep in mind. Personally I'd rather know that 100k mi on my odometer is actually 100k of milage on my car instead of 110k, or know that if I have 10 mi to empty that I can actually go 10 miles; rather than have the speedometer needle match what my GPS says.

 

If you're using the aspect ratio to calculate the tire wall height , those are nominal numbers and usually don't yield the correct height. I'm currently running 275/35/20 and if I calculate the revs/mile using the aspect ratio I get 731. If I lay a ruler on the tire and measure the actual tire diameter I get 750. I have GPS in both my head unit and radar detector and the computer speed reading on my scan gauge matches both units to within 1 mph, which is about as accurate as you can get since there are no decimal places. Of course you need to be on the highway at a constant speed , since the GPS doesn't change it's speed as fast. On the speedometer it's hard to tell how close it really is , since there are no mph hash marks , but it's pretty close.

 

Mystery solved. The Bama guys really came through in a big way. Ryan figured it out. The tune was for 3.31 gear ratio but I have 3.55. Once I installed the updated tune speedometer is good to go.

 

Good deal, glad it worked out!