I really dig the Desert Runner wheels but my '02 SE has 16's and the DR's only seem to come in a 15" size. Of course the 15's will fit but I wasn't keen on the idea of my speedometer being off.

The tire size calculator - thoughtfully provided at the top of this forum - says the speedo diff using 265/70/15 tires would be 3 MPH. But when I changed the tire size to 265/75/15, the speedo diff dropped to an insignificant .1 MPH.

I trust the calculator, but only to a point, and wanted to ask those with advanced tire/wheel knowledge whether 1) the calculation seems accurate and 2) given that a 75 will be taller and more narrow, whether in your humble opinion the height/width difference between the 70 and 75 aspect ratios is enough to be undesirable.

Thanks for any insight.

Not sure - but can you double check your figures on the calculator? Also, if you have 16 inch rims, how could you fit a 15 inch tire on it?

By no means am I an expert.

I thought that if you went with a higher aspect ratio (e.g. 65 to 70) that would increase the percentage that the speedo is off.

Example - start with P255/65/16

to
P255/70/16 - off by 3.5 percent

to
P255/75/16 - off by 7.1 percent

Sorry, I didn't explain fully. I meant if I got the 15" DR rims and mounted the 265/75/15 tires on them, not on my stock 16's.

The calculator result says the 265/75/15 tire/15" DR rim combo ends up close to the same overall diameter as the 265/70/16 tire/16" stock rim combo. That's what I'm asking about.

If your original tire is 265/70/16 it measures to 30.61" and with the 265/75/15 it would be 30.65", at 65 mph the difference would be .08 mph which is negligible IMO.

As far as the rims are concerned he has 16" rims and would like to get the 15" rims.

I see some misconceptions at the heart of this question.

Changing rim sizes doesn't necessarily mean that your speedo will be off. The only thing that will change the reading of the speedo is the outside diameter of the entire tire and wheel package. That is why a 265/75/15 shows up so negligable.

Also changing the aspect ratio doens't mean anything concrete. It will change the dimensions of the tire, but it won't affect your speedo in any way since the aspect ratio is a measure of the proportion of the sidewall in a percentage.

A 235/85/16 is nearly the same height as a 265/75/16. The only difference is that there is 20% more sidewall despite the smaller size of the tire it is just as tall. Instewad of a 31.5x10.5 you get a 31.5x9.5

Also, one last thing, your speedo is not going to be off a universal 3mph or .8 mph. The error in your speedo should be expressed in a percentage. If you were off 10% then you are off by 3mph at 30 mph and off 6mph at 60 mph.

So, if you're trying to not change your speedo than a 31x10.5x15 tire will get you right where you want to be. It will be slightly larger but thats ok cause you speedo will read slightly slow and your odo will turn fewer miles. A 31x10.5 will put you 2mph off at 60 mph which is 3.3% error. You will never even notice. THats 1mph off at 30 mph and if you want to extrapolate than you're only 3mph off at 90mph which won't matter cause you'll get pulled over anyway if your X hasn't driven itself out of control.

ps. good luck finding a 265/75/15. I think I could on;y find 2 companies that make that size, but it will be an exact match for yout stock size.

Well, it's not a misconception but what you say otherwise is pretty accurate. If you just change the rim size, and use a tire of the correct size for the new rim, then your speedo will be off since it's calibrated for the stock combination. And that was the point of my question, to verify that using a 75 aspect ratio tire, which is taller, would compensate for the smaller 15" rim and make the overall rim/tire diameter close to the same as the stock 70 aspect ratio and 16" rim.

Just go to the tirerack site (Banner above)

and find the tires you want, go to specs for that tire, and see the diameter.

A 30" diameter tire for a 15" rim will be the same diameter as a 30" tire on a 16" rim, etc.

You can also see the section on "Plus 1", Plus 2, sizing, etc....



The aspect ratio is the relationship betwen the section width, such as 265 mm, and the sidewall height....with the lower aspect ratio having a shorter sidewall height....for the same section width.

So, yes, you can keep the same overall diameter by getting a wider section width but lower apect ratio, or, a narrower section width and a higher aspect ratio.

There is no such thing as one "Correct tire size for the rim to provide an accurate speedometer"....you can get it with lots of combinations.



You can, as described above, get any diameter tire you want on a given rim, within the obvious practical limits.

If you off road, you generally want the tallest fattest tires that can fit into the wells....to get more grip and flotation.

For on road, you want the fattest tread, but not the tallest, typically, you want the shortest diameter that you can stand comfort wise....to get the best handling.

(In your case, an easy bye would be to just get a 31x10.5/15 of course)

If you must go metric...

For any particular tire, a 70 series (aspect ratio) tire handles better than a 75 series, as the 70 series has a shorter sidewall, and will be less wishy-washy....even though the overall diameter may be the same.

I have 305/70/16's BFG AT ko's on my '01 X....they are an even 33" diameter, and have a tread width a bit larger than a 33x12.5/15's.

Some people have 285/75/16's...which are essentially like a 33" diameter 11.5" section width tire.

So - we have an example of a 305/70 being the same diameter as a 285/75....

So - as you can see, you can create any combination of width and height you need, by playing with the aspect ratios and section widths...and as long as you know what you WANT, you can find a tire that gives it to you.



I hope some of this helps.

To add....you won't find a 265/75/15 tire size. What you will find is a 31x10.50/15, which is basically the same thing. I made this same swap on my X (not DR rims, but 15" polished Frontier rims instead. The tires were the same height.