I have an SCT X3 will a tune from Don at LaSota Racing. I am having troubles getting the speedometer correct with the correct Revs per miles.

I have 265/75 r16 tires so the calculated Revs/Mile is 637.6. Now I know all tires are slightly different and there is the 'squish' calculation so I did some playing around on my last road trip with a GPS and am now at 645 on the tuner, which yields a odometer error of 1% error for everyone 100 miles (reads 99.5 at 100 miles). Close enough.

The problem is the speedometer is off by 2 MPH around 60 and 3 MPH around 80. I have tried 2 different GPS.

Anybody know why this is happening? It's not the biggest issue in the world but if I am able to fix it I would like to. It doesn't make sense to me because the odometer and speedometer should match each other.

Also, don't know if it matters but I changed from 3.55 gears to 4.10 gears but I DID adjust that as well on the tuner.

Thanks guys

 

thats an error of less than 2%. thats just getting too picky.

and when your tires wear, that will change again as the diameter changes.

playing with the rotations per mile is the only way to change that.

 

Are the odometer and speedometer separate sensors? Because I can adjust the Rev/Mile so that either one is correct, but one is off if the other is correct.

 

Yep, sounds well within 'factory' tolerances & degree of accuracy to me.

EDIT:

According to good ol' Wikipedia:

"Most speedometers have tolerances of some ±10%, mainly due to variations in tire diameter. Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits. Federal standards in the United States allow a maximum 5 mph error at a speed of 50 mph on speedometer readings for commercial vehicles."

 

Interesting, sounds like I am in the legal and normal limits. Makes sense that my speedometer is reading a little high then.

 

I'm an engineer at Kia Motors and our spec is within about 2 mph at 60. So that's pretty good.

If you want to be dead nuts on your gonna have to drive with the cluster lens off and pop the needle off and reset it at the correct speed.

Most cars are off by 1-2 mph from factory.

 

Im sorry for your loss.

 

How so?

 

1-2 mph is fine. I usually drive 5 over anyway, idk how off my speedo is but I really don't care lol. I may check it in the spring and mess with my tuner.