Tire size calculator
Enter your current tire size and the one you're thinking about. The calculator works out sidewall height, overall diameter, circumference and revs per mile for both, plus what your speedometer will read. Keep the overall diameter within about 3% of stock. Past that, plan on recalibrating the speedometer.
Result
Fix the highlighted box to update the results. Width 105 to 455 mm, aspect ratio 20 to 95, rim 10 to 30 in, speed 5 to 200 mph.
| Current | New |
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The 3% line comes from Discount Tire: keep overall diameter within about ±3% of the original size. It recommends recalibrating the speedometer any time you change tire size.
How to use the tire size calculator
Read the size off the sidewall of your current tire and type it into the first box. Then type the size you want into the second. A size like 225/45R17 breaks down into three numbers: width in millimeters, sidewall height as a percentage of that width, and rim diameter in inches. Our guide on how to read a tire size covers the letters and the load and speed codes.
Results update as you type. The speed box sets the speedometer check, 60 mph by default.
How the math works
A tire size has everything you need. The sidewall is the width times the aspect ratio. The overall diameter is the rim plus two sidewalls. From there it's circle geometry.
Diameter (in) = rim + 2 × sidewall ÷ 25.4
Circumference (in) = π × diameter
Revs per mile = 63,360 ÷ circumference
There are 63,360 inches in a mile, which is where that last number comes from.
Here's the default comparison, 225/45R17 against 245/40R18:
| 225/45R17 | 245/40R18 | |
|---|---|---|
| Sidewall | 101.3 mm (3.99 in) | 98.0 mm (3.86 in) |
| Overall diameter | 24.97 in | 25.72 in |
| Circumference | 78.45 in | 80.79 in |
| Revs per mile | 807.6 | 784.2 |
The new tire is 2.98% taller. That's just inside the 3% line. Each turn of the new tire covers more road, so the car goes farther than the speedometer thinks. When it shows 60 mph, you're doing about 61.8 mph. Speedometer error is a straight ratio of the two diameters: true speed = shown speed × new diameter ÷ old diameter.
The width grows 20 mm, so each side sticks out about 10 mm more, inside and outside.
What the numbers mean
The diameter difference is the one to watch. Discount Tire says to aim for within about ±3% of the original size. It also recommends recalibrating the speedometer after any size change. A taller tire makes the speedometer read slow. A shorter one makes it read fast. Treat 3% as a common guideline, not a law.
Plus sizing is how you stay inside it. Tire Rack's rule of thumb is to add 10 mm of width and drop the aspect ratio 5 to 10 percent for each inch of rim. Done well, the diameter changes by only a few tenths of an inch.
Revs per mile matters for gearing. Fewer revs per mile means taller effective gearing: lower engine speed on the highway and a little less punch. Our gear ratio calculator shows the effect gear by gear.
Comparing a few candidate sizes? Run each one against your stock size and note the percentage. All else equal, the one closest to zero keeps your speedometer, gearing and ride height closest to how the car left the factory. Then pick between the close ones on width, price and what's actually in stock.
The width per side is a starting point for clearance. Add it to the poke from the wheel offset calculator if you're changing wheels at the same time.
What this calculator can't tell you
It uses the nominal size printed on the tire. Real tires differ by brand and model, and the same tire measures differently on a narrower or wider rim. Check the maker's spec sheet for the measured diameter and section width.
It assumes a metric (P-type or Euro) size. Flotation sizes like 33x12.50R15 give the diameter directly, so you don't need this math for them. Tread wear also shrinks the diameter as the tire ages, so a new tire against a worn one will look a little different on the road.
Finally, it can't check load index, speed rating or whether the tire clears your suspension. Those come off the sidewall and the door-jamb placard.
Sources are linked in the text. Main references: Discount Tire and Tire Rack.