Help please
To ajust tire size the programmer is asking for Tire Revolutions per mile starting at 500 going up 5 at a time , I have 35+12+50 tires ?
How do I calculate this?

 

I've never seen a tire size like yours. Are you sure that's what you have? Look on the sidewall for something like:
275/65/R18 (which is what mine are)
or something with a similar "format".

If you have no luck, the next approach is to mark your tire at the center point of its contact with the ground and put a mark on the pavement there too. Then, move your truck forward or backward until the tire mark is again "centered" on the ground.

Measure the distance between this point and the mark on the pavement you made earlier.

For an example calculation, suppose the distance you measured was 100.5 inches. This is the distance the tire moves in one revolution. There are exactly 63360 inches in a mile, so 63360 divided by 100.5 is 630.4 revolutions per mile.

Pick an increment that is closest to what you calculate. This is not rocket science, just grade school math. And, be prepared to start "adjusting" in "5 revolution increments" to make your speedo read correctly. What you measured will not really quite match what happens on the road.

If your speedo is slower than exact, you'll need to add "revolutions". If it is faster than you are really going, you'll want to subtract revolutions.

- Jack

 

Thank you for your help.........

 

35s should be around 595 revs per mile. However when you get into a tire that big there tends to be some variability based on tire manufacturer, wear, and inflation pressure. You may want to "dial in" your speedo with a GPS and make small changes to the revs per mile values.

 

Thanks Justin , its pretty close just needs a small ajustment the 595 you provided is the # I was looking for Thanks

 

I'm looking at 255/75R17 & 285/70R17.

Anyone know what those RPM #s are?

 

http://www.moparregister.com/articles/calc/


According to this site, your Revolutions per mile should be around 629. for the 255/75/17.

285/70/17 ='s approx. 616 rev's per mile.


I used this to figure out my 265/70/17's and it got me in the ball park. Like many other guys have mentioned, you'll want to play with it a little to get it dead on.

Good Luck!
Tom