Hey guys,

I just got my new wheels and tires (pics to come soon) Anyway, I am trying to adjust my speedometer with my X Cal 3. I went from a 275/55/20 tire to a 275/70/18 with a 18x9 wheel. I am trying to figure my revs per mile so that I can input it. Trouble is, when I've gone to a few sites with tire calculators, I am not getting the same results. What's up??? How do I figure this out?

Also, to those of you with X Cal 3's Can you onlt adjust the revs by increments of 5 only? I did not see anywhere where I could change the digits by any less than that. I hope somebody can chime in and help

 

Go to your tire manufacturer's web site. Nearly everyone lists the rev's per mile for their tires. If not, go to another manufacturer who makes a tire in that same size and check their info.

 

To calculate the tire diameter, take the width size (275 mm) and multiply by 0.03937 to get inches.
Then take the ratio (55%) and multiply to the width to get the sidewall height.
Then take the wheel diameter (20 in.) and add the sidewall height times 2.

This will give you:
275/55/20 = 31.91 in. dia.
275/70/18 = 33.15 in. dia.

This transulates to:
275/55/20 = 632.04 rev. per mile
275/70/18 = 608.25 rev. per mile

 

Only problem with actually doing the math is that you're going by the tires rated OUTSIDE DIAMETER......not that STATIC LOADED RADIUS....which is a little different. Also, tires aren't always exactly the same size that's written on the sidewall. Revs per mile is based on the S.L.R. not O.D. Using a tire calculator could result in a 10% error.