Tuning Help Pt. 2

[Cris Laroco]

I just had my XCal2 retuned and it looks like I'm still having some issues with my speedometer not reading correctly. It constantly reads anywhere between 2-4 mph faster than I'm actually traveling. Talking with Torrie over at Fast Parts Network, he says that it's more than likely the tire revolutions per mile that is incorrect. I'm running Pro Comp All-Terrains 285/75/16s which comes out to just over 614 revolutions (thanks again Dingo for the math lesson). We're going to try and play with the number of revolutions and I was wondering if any of you with the same tire size have had this problem. If so, what number are you using for the revolutions per mile? Should the number be higher or lower than 614 to get my speedometer to read correctly?



Thanks again for any help.

 

[Scott Heckler]

When I did the math I came up with 613. I think that is pretty close. (32.9 x 3.1415 = 103.35535 then 103.35535 divided by inches in a mile 63360 = 613.0306) I was sooooo far off before I got Torrie's tune. If I am off I think I would be going a mile to a mile and one half slower at 75 - 80. I gave Torrie 613. Not sure if his program would need or take the .0306 and if that would be that much of a difference.



I also have the 4.11 rear if that makes any difference too.

 

[Todd Z]

How are you comparing how fast your going ??? A car next to you ??? Police gun ???



All speedo's do read 3-5 MPH off, none are 100% accurate..



Todd Z

 

[Cris Laroco]

I'm using my Eclipse GPS unit to get my actual speed. Because I work for the city, I have access to the sheriff's station and their radar guns. I had an off-duty officer help me check and see how accurate the GPS info was when I first got it and it was right on, so that's what I've been using as a speedometer.

 

[A K]

Agree with the above. I went from P255s to P275s. Now, my speedometer is fully acurate per the GPS's speedometer. The P255s constantly read about 4-5 MPH below my actual speed at 65-70.

 

[Cris Laroco]

Hey Scott - Do you think that dropping the revs per mile down to 613 would help any? I'm assuming that because it's just one less revolution that it won't make much difference.



And as for the speedo, I drove it around and noticed that when the speedo says I'm going 40mph, I'm really traveling at 37mph. When it says I'm going 50, I'm actually traveling right between 45 and 46mph. I haven't had a chance to get on a freeway and check those speeds, but I'm going to guess that it just gets worse.



But anyways, do you think I should have Torrie increase the revs per mile or decrease it? And by the way...Torrie is an awesome guy to deal with. I've had a few issues and tons of questions and he's been very good about getting me answers very quickly.

 

[Torrie McPhail]

Getting the RPM right is critical to speedo accuracy. Test file sent.





Take care,

Mr. Torrie R. McPhail - (Owner)

http://www.FastPartsNetwork.com

http://www.UnleashedTuning.com

Ford/GM/Dodge/Chrysler/Jeep SCT Custom Calibrator

 

[Cris Laroco]

Thanks Torrie! I'll try it as soon as I get home and let you know how it goes.

 

[Dan Long]

The overall diameter of your tire will vary depending on how wide of a rim its on, how much tread is left and what tire it is. Even though the size calculations seem like they should be right, if you look at different tire specs for the same size, they can vary almost 1/2" between tires.



The best way is to do a rolling chalk test. On a flat, hard surface make a chalk mark on the tire tread. Roll the vehicle until the chalk leaves two marks on the ground, then measure between them. This will give you your actual rolling diameter of the tire with all the variables taken out, convert that to RPM's and your set. The only thing this doesn't account for is tire deflection from the centrifugal force, but its as close as you're gonna get.

 

[Cris Laroco]

Dan - That was actually going to be my next step if the tune I have waiting for me at home is still off. Thanks for the advice, though.