Ok so I just got some new rims and tires in the rear, they are 18 x 10's. Now I was wondering and have heard that I have to change something with the SCT tune because I had 17 x 8 rims before. What is it that I have to do? Thanks.

 

The width of the rim doesn't matter, it's how tall the tire is. You need to adjust the speedometer fora taller or shorter tire than stock. You can go online and put in your tire size and it will tell you how many revolutions per mile then you can adjust that with your tuner. http://www.1010tires.com/TireSizeCal...?action=submit

 

I'm staying at 17s but diff tire size 235/550 to 255/50. This calculator shows speedo will be .522% too fast. i assume I don't need to do any speedo changes for something that small, right?

 

Off by 1/3 mph at 70. I don't think you need to do anything.

 

Close enough for me. Here is one of a number of calculators I found http://www.csgnetwork.com/speedocalibcalc.html

 

i go back and forth between 18" winters and 19" summers; i haven't adjusted anything w/. my xcal, though. i wonder, is this necessary? i plugged in some numbers and simply changing from 17" -> 18" seems to result in a 5mph underestimation by the speedomoter, so now i'm worried...

 

What size are the 18" tires and what size are the 19"?

 

the two sets are as follows:
winter = 235/50/18
summer/front = 245/40/19; summer/rear = 275/40/19

i just hit up that calculator, and even at 75 mph, it only seems to beabout 1mph off.

but then again, i didn't compare them to the stock 17"s since that's what the speedomoter was originally calibrated to - hmmm, maybe it's even worse...

 

How about using a GPS system. It tells you how fast you are going.
Compare that speed to speedometer on your car? Just an idea.....

 

alright, just checked the calculator again, and the numbers are negligible imo. comparing the winter set and the summer set to the stock set, the worst the speedomoter is ever off is +/- 1 mph. i think i can live with that