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.Originally Posted by GPintheMitten
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