Tire Size Calculator: metric tire math made useful
Because apparently millimeters × a percentage + wheel inches was the clearest way to label a tire. Compare any two sizes and get the numbers that actually matter.
The short version
Enter what you have and what you want. We'll turn the sidewall hieroglyphics into actual diameter, clearance, speedometer error, and gearing impact—then show the axle ratio that gets your stock performance back.
The de-confuser
Put two tires head-to-head
265/70R17 · ≈ 31.6x10.4R17 flotation
285/75R17 · ≈ 33.8x11.2R17 flotation
Same ground. Different consequences.
Diameter
+2.2"
31.6" → 33.8" (+7.0%)
Width
+0.8"
10.4" → 11.2"
Ride height
+1.1"
at the axle (half the diameter change)
Sidewall
+1.1"
7.3" → 8.4"
Speedometer reality check
69.6 mph actual
Bigger tires: you travel faster than the speedometer shows · 596 vs 638 revs/mile
Gearing gut-check
3.48 effective
Re-gear to ≈3.99 to restore stock performance
Computed from nominal sizes. Mounted dimensions vary by brand, load range, and rim width — check the manufacturer's spec sheet before buying.
What the numbers mean
The four things your sidewall refuses to explain
Diameter is the number that matters most. It sets your ground clearance at the axles, how the truck gears feel, and how far off the speedometer reads. Sizes within about 3% of stock behave like stock; beyond 8–10% most owners budget for a re-gear.
Width decides flotation in sand and snow — and whether the tire rubs. A taller narrowtire often clears where an equally tall wide one won't, and does better in ruts and on fuel economy.
Speedometer error compounds quietly: GPS apps, tolls, and your odometer (hence resale mileage and maintenance intervals) all drift with tire diameter. Recalibration is cheap on most platforms.
New to the notation itself? Start with our guide to how to read a tire size, then use the calculator to compare candidates for your build. Or skip the math entirely: the truck tire size chart lists every common size from 31 to 37 inches with its real height and width, and each size has its own page with the tires in stock in it.
Frequently Asked Questions
What is the difference between metric and flotation tire sizes?+
Metric sizes like 285/75R17 give the section width in millimeters (285), the sidewall as a percentage of that width (75), and the wheel diameter in inches (17). Flotation sizes like 35x12.50R17 state the overall diameter (35") and width (12.5") directly in inches. Neither is "bigger" by definition — a 285/75R17 and a 33.8x11.2R17 are the same tire described two ways.
Do bigger tires make my speedometer wrong?+
Yes. Your speedometer counts wheel revolutions calibrated to the factory tire diameter. A larger tire covers more ground per revolution, so the speedometer reads lower than your true speed — roughly 3% low for a 3% larger tire. Many trucks can be recalibrated with a handheld tuner or by the dealer.
When do I need to re-gear after a tire upgrade?+
Larger tires reduce your effective axle ratio, which costs acceleration and can push highway cruising out of the engine’s happy RPM range. As a rule of thumb: up to ~3% diameter increase is rarely noticeable, 3–8% is a judgment call (towing and automatics feel it more), and beyond ~8–10% most owners re-gear to a ratio that restores the factory effective gearing.
Will a bigger tire size fit my truck?+
Diameter is only part of fitment — width, offset, backspacing, and suspension travel decide whether a tire rubs at full lock or full compression. Check a vehicle-specific fitment guide for your platform; small lifts, wheel spacers, or minor trimming are often part of the recipe for popular sizes.
How accurate are these numbers?+
The math is exact for the nominal size, but real mounted tires vary slightly from nominal — typically within a few tenths of an inch of diameter — depending on brand, load range, rim width, and inflation. Use the manufacturer’s spec sheet for the final check.