Tire Size Calculator
Two tire markings in — diameter, width, revs per mile and the speedometer error of the swap, side by side.
The short answer
Formula. Overall diameter (inches) = wheel diameter + 2 × (section width in mm × aspect ratio ÷ 100 ÷ 25.4); revolutions per mile ≈ 63,360 ÷ (diameter × 3.1416).
Example. A 225/65R17 tire stands 28.52 in tall; upsizing to 235/65R17 adds 1.79% rolling diameter, so the speedometer reads 60 mph at a true 61.07.
Compare sizes
Format: 225/65R17 (letters like P or LT in front are fine)
How tire size math works
A marking like 225/65R17 packs three numbers: the section width in millimeters (225), the aspect ratio (65 — the sidewall is 65% of the width), and the wheel diameter in inches (17). The overall diameter is the wheel plus two sidewalls: sidewall = 225 × 0.65 = 146.25 mm = 5.76 in, so the tire stands 17 + 2 × 5.76 = 28.52 inches tall. Circumference is π × diameter, and revs per mile is 63,360 ÷ circumference — the number that drives your speedometer and odometer, which count wheel revolutions, not road speed.
That is why upsizing fools the gauge. A taller tire covers more ground per revolution, so the car travels farther than the speedometer believes: the 235/65R17 (29.03 in) against the 225/65R17 (28.52 in) is a 1.8% diameter increase — at an indicated 60 mph you are doing 61.1, and every 10,000 odometer miles undercounts by about 180. Wider tires also add rolling resistance and can tramline; taller effective gearing slightly dulls acceleration but can drop highway RPM. The calculator quantifies all of it from the two markings.
What the numbers cannot see: fitment. A wider tire on the same wheel can rub fenders or suspension at full lock; a taller one may foul the liner over bumps; load index and speed rating must meet or exceed the original; and staggered setups (wider rear) have their own rules. Plus-sizing convention — wider tire, bigger wheel, shorter sidewall, same overall diameter — exists precisely to keep the speedometer honest; anything inside about 1% overall diameter is effectively invisible to the gauge, and past 3% you should have the speedometer recalibrated (many cars can be reflashed). Verify any swap against your vehicle's placard and a fitment guide before buying.
Sources & review date
- Markings follow the standard tire-size convention: aspect ratio is sidewall height as a percentage of section width; 25.4 mm = 1 inch, exact.
Last reviewed: October 2026. Numbers that drift (fees, prices, densities) are editable inputs on this page.
Tire size FAQs
Can I use 265/70R17 instead of 265/65R17?
Run it through the calculator: the 70-profile is 1.46 in taller (31.61 vs 30.56 in), a 3.4% change — over the usual 3% guideline. Speedo and odometer will read low, and clearance is the real risk. It fits some trucks, not others.
Which number matters most for speedometer error?
Overall diameter. Width alone (225 vs 235 at the same aspect and wheel) barely moves it — aspect ratio and wheel size do the heavy lifting through the sidewall height.
Do wider tires hurt fuel economy?
Usually slightly, through rolling resistance and mass — more width means more tire on the road. If the swap also raised diameter, the lower highway RPM can claw a little back. Expect small changes, not transformations.
What do the extra letters mean (P, LT, XL)?
P designates a metric passenger tire, LT a light-truck tire with a stronger carcass (and usually higher pressure range), XL an extra-load rating. They do not change the size math, but load capacity matters for trucks and vans.