I'm running 275/40/18 Tires, so I went to my Xcal2 to run the tire size correction.

According to this calculator:
http://www.miata.net/garage/tirecalc.html

With 275/40/18 my speedo should be about 2mph fast, which matches what I see on my GPS.

I tried a couple of online calculators and they all give me 756 revs/mile for this tire size, I input this into the Xcal, and my speedo is off even more, now my speedo is indicating about 7-8mph faster than I'm actually going.

Any ideas?

 

I have 285/30/20 on the rear and when I calculated the diameter using these numbers I got 26.7323" which gave a rev/mi of 754.45 and my speedo was off by about 3 mph. So I physically measured the diameter and it was 26.5" which yielded 761.1 and it now matches my GPS. I guess sometimes the tire sizes aren't exactly what they say ...

 

Yes, actually measure the tire.
Then (if need be) fine tune it, adjusting until you match the GPS speed.

 

I suppose that's the easy answer, thanks guys. One more question, I'm currently running 275/40R18 all around, but this weekend will be swapping the fronts for 255/45R18 (can't deal with the tracking/rubbing in the front any more), which set of wheels are the speedo readings based on?

 

My tuner told me you have to go to the manufactors specs and see what they list for revs per mile. For instance My Toyo STII are 769 revs per mile:

255/45R18 99V

769

http://toyotires.com/tire/pattern/proxes-st-ii

 

The rear wheels ...

 

Thanks for the tip, looked up my Falken FK452s and the revs/mile comes out as 781 instead of 756, hopefully that makes the difference.

 

If you do the math using the tire size you get 275 x .4 = 110 mm x .03937 = 4.3307" x 2 = 8.6614" + 18" = 26.66" and that is the tire diameter. Then 20168 / 26.66 = 756.5 rev/mi. Like I said mine calculated out to a diameter of 26.73" and a rev/mi of 754.5 but the speed was off. The best way is to actually measure the tire diameter , I just put a ruler on top and measured to the ground , and then divide 20168 by this dimension and you should get the correct speed ...

 

Why do this when you can find it easily on the internet and be exact. Then you don't have to worry about the accuracy of the measurement, was the tire inflated properly, etc.

Jedikid,
NP, glad I could help. Let us know if it works.

 

Takes me back to my mechanical engineering classes...

The odd thing is, on the Falken website, they list 26.6" diameter and 781 revs/mile, that doesn't seem to match. I got the same #s as you doing the long-hanged way. (5280/(26.66*Pi/12))= 756.49 revs/mile.

Weird.. I'll try it with 780 and measuring the tire and see how it works out.

 

I actually like doing the Math because I'm a engineering geek, haha.

In school I made a excel file that correlated engine RPM to vehicle MPH based on this Yamaha R6 engine we were using for a project, it was pretty cool trying it out and using internet websites that list these values as a check

 

I think we are all geeks in one way or another on this site.

I do programming which involves a lot of date stuff. It can be a real pain doing that sort of math.