Compare any two sizes — metric (285/75R17) or flotation (35x12.50R17) — and see exactly what changes: diameter, width, ride height, speedometer error, and your effective axle ratio.
How it works
Enter your current size and the size you're considering. Diameter drives everything: a bigger tire lifts the truck by half the diameter change, makes the speedometer read low, and reduces your effective gearing — the calculator shows each effect and the axle ratio that would restore stock performance.
265/70R17 · ≈ 31.6x10.4R17 flotation
285/75R17 · ≈ 33.8x11.2R17 flotation
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 error
69.6 mph actual
Bigger tires: you travel faster than the speedometer shows · 596 vs 638 revs/mile
Gearing
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.
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.
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.
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.
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.
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.
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.