So I went to the dealership...

[GPintheMitten]

Regarding the tire sizes, you have a 32" tire (diameter). Any 32 inch diameter tire on his options would get the speedo calibrated. The 245/60/18 is the same diameter as stock which is 29.5 inches.

Here's a tire calculator link.

http://www.discounttire.com/dtcs/infoTireMath.dos

[07JeepXK]

Regarding the tire sizes, you have a 32" tire (diameter). Any 32 inch diameter tire on his options would get the speedo calibrated. The 245/60/18 is the same diameter as stock which is 29.5 inches.

Here's a tire calculator link.

http://www.discounttire.com/dtcs/infoTireMath.dos

One thing to consider. If you are running a 255/75R17 for example and the tire calculators say its 32 inches tall, you dont want to program 32 inches into your programer to correct the speedo. You need to actually measure the tire because its not going to be that tall when mounted on a vehicle. The best way to find out the diameter of a tire is to find the tires circumference and then divide that by pi (3.141593). This will give you the most accurate diameter since you are getting the rolling circumference of the tire. To do this, take a piece of chalk and draw a line down the side wall of the tire (perpindicular to the pavement) and continue that line onto the pavement. Then drive your vehicle forward so the line on the side wall does one complete rotation and is perpindicular to the pavement again. Draw another line on the concrete where the line on the sidewall touches the concrete and then measure the distance between the two lines on the concrete. This will give you the tires circumference. Then take that number and divide it by pi and youll get the actual rolling diameter. I have 265/70R17 on my XK which according to Goodyear are approx. 31.9 inches tall brand new. They are fairly worn now but when I measured them the other day using that method it came out to be 30.5 inces tall. I pluged that number into my superchips programmer and now my speedo is dead on to my GPS.

[NeilSmith]

i think someone here should by a starscan and when a forum member needed something done the starscan would be Fed-exed to him, and then sent back after work completed. Sounds good , huh ?

[07JeepXK]

That would be cool until it got sent to someone and that person never returned it.

[NeilSmith]

Well, thats just a chance YOU will have to take. LOL

[CrunchyWizard]

Has anyone with larger than stock tires managed to get the dealership to adjust for them, or is it (as it seems) only possible through Superchips?

I just wonder if anything other than MPH and MPG calculations are thrown off... Like traction control, ABS. The transmission is going to shift later since it thinks the XK is going ~10% slower than it really is.

[07JeepXK]

I know there was a few members from jeepcommander.com that got their tire size adjusted by the dealer. They can only program certain sizes though like you mentioned in your first post.