DML Tire and Wheel Calculator


Quick Converter
(Requires Javascript)
P-Metric Style

Example: 215/75R15
/ R
Copy values to:

Inch Style

Example: 31 x 10.50 x 15
x x

Information about how to use this program can be found in the chart at the bottom at the page.
Tire A:

Tire B:

Just calculate Tire A's dimensions
Compare Tire A to Tire B
Calculate tires equivalent to Tire A (Use Tire B fields to narrow search). Max height difference: inch(es).

Fill in the following if you're interested in calculating gearing and/or RPM changes
Rearend ratio:     RPM:

Tire A: / R

Tire B: / R

Just calculate Tire A's dimensions
Compare Tire A to Tire B
Calculate tires equivalent to Tire A (Use Tire B fields to narrow search). Max height difference: inches.

Fill in the following if you're interested in calculating gearing and/or RPM changes
Rearend ratio:     RPM:

Welcome to the DML Tire and Wheel Calculator! This program is designed to help those of you who want to change the size of your wheels and/or tires. Hopefully, it will help you to sort through the seemingly infinite number of wheel and tire combinations. There are a few things you can do:

Here's how it works:

This form is divided into two sections. They are both the same except that the one on the left uses drop down listboxes so that you can just select the data rather than having to type it in; the section to the right uses text input boxes instead. The drop down listbox version is probably easier to use for most people, but the text entry option is still there in case you need to check a tire which isn't available from the default selections.

How you use this program depends on what sort of information you're looking for. The following chart should help you out:

If you want thisThen do this
The dimensions of your current tires. (Or any tire, for that matter...) Input your tire as Tire A, select Just calculate Tire A's dimensions, and hit the Submit button.
To compare the dimensions of two tires. In addition, this will give you the speedometer error and can also calculate the effective (rearend) gearing change and RPM difference if you were to go from Tire A to Tire B. Input your current tire numbers as Tire A, input the numbers for the proposed tire as Tire B, select Compare Tire A to Tire B, and hit the Submit button.
If you are interested in the effective rearend gearing change and/or the change in your RPM, input that data too. (The rpm difference is useful if you are trying to get an idea of how a change would affect your gas mileage (cruising RPM), or if you are a drag racer, you might be interested to see how your finish line RPM would change.)
To determine which tires you can use to maintain your current gearing ratio and keep your speedometer accurate. Input your current tire numbers as Tire A, select Calculate tires equivalent to Tire A, input an acceptable max height difference, and hit the Submit button. You can use the fields of Tire B to narrow your search. For example, if you are only interested in 15 inch rims, select a 15 inch rim for Tire B, and the program will only calculate for 15 inch tires.
Max height difference: use this field to determine how "close" of a match you want. For example, if you only want to see tires within 1/4" of the height of your current tire, input .25. (the default is 1/2 inch) In the example above, if your current tires were 28 inches high, the program would only consider a tire a "match" if it was between 27.75 and 28.25 inches high. (Entering 0.00 here would require that a tire be a perfect match, which probably isn't gonna happen...)
When the program pulls up the list of compatible tires, there will be a button next to each one that you can press to calculate the difference between that tire and your current one. (Exactly the same as if you had entered the dimensions here and used the Compare Tire A to Tire B option.) Therefore, if you will want to compute rearend and/or RPM differences, input that data NOW, since you won't be able to do it later.

Note: Just because this program says a that certain tire is a "match" doesn't mean it'll fit on your vehicle!!!! If you drive an econo-box that came with 160's, you will have a difficult time trying to stuff 325's on all four corners, even though there is probably a 325 in a ratio that would keep your speedometer accurate... :-) There are a lot of other things to consider like backspacing, offset, rim width, wheelwell clearance, etc. But this should get you in the ballpark...
Also, there's no guarantee that a particular tire size is even made... Some sizes can be difficult (or impossible) to find.
Any of the above, but in "inch" format, instead of P-Metric Due to the way the calculator is written, all calculations are based on the P-Metric sizing. However, you can still use this calculator to select "inch" style tires. First, use the "quick calculator" at the top of the page to convert the inch size you are interested in into P-Metric format, then input those numbers into the DML Wheel and Tire Calculator. Whenever the calculator gives you a tire size in P-Metric format, it will also display the equivalant "inch" style size next to it. You will notice that some of the sizes will be slightly off also - for example, if you use the quick calculator to find the metric size for a 33x12.50x15 (318/72R15) and input that into the calculator, you will find that it reports the size of the original tire is 33x12.52x15. A tad annoying, but its due to rounding error, and won't actually throw off the calculations by any noticible amount. Its a bit of a round-about way to do it, but this was the fastest and easiest way to add this functionality within the time limits I have to work in...

Note: I believe most (4x2) Dakotas come off the assembly line with 215/75 R15's, so I've made that the default selection for Tire A.
Note: The Javascript "quick calculator" at the top of the page is based on code from http://www.4lo.com, written by Mark I. Medina. Because I don't know javascript, I grabbed his code and used it as a starting point. As requested in his license, I am including this blurb to give him proper credit.
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Jon N. Steiger / jon@dakota-truck.net