How do I read a tire size like 225/45R17?

225 is the section width in millimeters, 45 is the aspect ratio — the sidewall height as a percentage of the width (45 % of 225 mm = 101 mm) — R means radial construction, and 17 is the wheel diameter in inches. A leading P (P225/45R17) marks a passenger tire; LT means light truck.

How much bigger can I go on tire size?

The common guideline is to stay within ±3 % of the original overall diameter. Beyond that the speedometer and odometer drift noticeably, ABS and stability-control calibration is affected, and the tire may rub the fender or suspension at full lock. Trucks lifted for larger tires often need a speedometer recalibration.

Does a bigger tire make my speedometer read low or high?

A larger overall diameter covers more ground per revolution, so the car is actually going faster than the speedometer shows: on tires 3 % larger, a reading of 60 mph is a true 61.8 mph. Smaller tires do the opposite — the speedometer reads high and the odometer over-counts miles.

What is a plus-size (plus-one, plus-two) tire fitment?

Going up one inch in wheel diameter while lowering the aspect ratio and widening the tire slightly, so that the overall diameter stays the same: 205/55R16 → 225/45R17 → 235/40R18 is a classic plus-one/plus-two ladder. The calculator confirms the diameter difference stays near 0 %.

How do I convert a metric tire size to inches?

Diameter in inches = rim diameter + 2 × (width × aspect ÷ 100) ÷ 25.4. For 265/70R17: sidewall = 265 × 0.70 = 185.5 mm = 7.30 in, so diameter = 17 + 2 × 7.30 = 31.6 in — the size often sold as a “32-inch” tire. The width is 265 ÷ 25.4 = 10.4 in.

Tire Size Calculator

Name: Tire Size Calculator
Author: Miikka

Enter your current tire size and the size you are considering — width, aspect ratio and rim — and see the overall diameter, sidewall height, circumference and revolutions per mile side by side, the percentage difference, and how far off your speedometer will read. Paste a size such as 225/45R17 to fill the fields automatically.

By Miikka · CalcBrew · Updated Sep 16, 2026

Current tire (stock)

Width (mm)

Aspect ratio (%)

Rim (inches)

225/45R17

New tire (compare)

Width (mm)

Aspect ratio (%)

Rim (inches)

235/40R18

Spec	225/45R17	235/40R18	Difference
Section width	8.86 in (225 mm)	9.25 in (235 mm)	+10 mm
Sidewall height	3.99 in (101.3 mm)	3.70 in (94.0 mm)	-0.29 in
Overall diameter	24.97 in (634.3 mm)	25.40 in (645.2 mm)	+1.72 %
Circumference	78.45 in	79.80 in	1.35 in
Revolutions per mile	808	794	-14

Diameter difference

+1.72 %

Within the usual ±3 % guideline.

Speedometer reads (mph)

Actual speed on the new tires

61.0 mph

How the tire dimensions are calculated

sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4

diameter (in) = rim (in) + 2 × sidewall

circumference (in) = π × diameter

revolutions per mile = 63,360 ÷ circumference

Worked example: 225/45R17. Sidewall = 225 × 0.45 = 101.3 mm = 3.99 in. Diameter = 17 + 2 × 3.99 = 24.97 in. Circumference = 78.4 in, so the tire turns about 808 times per mile. Swapping to 235/40R18: sidewall 3.70 in, diameter 25.40 in — +1.7 %, well inside the ±3 % rule, and the speedometer will read 60 mph when you are doing 61.0.

These are nominal figures from the size markings. Real tires vary by brand and model by a few tenths of an inch, and the loaded rolling radius is a little smaller than the unloaded diameter, so manufacturers’ published revolutions-per-mile numbers are typically 1–3 % higher than the nominal calculation.

Common OEM sizes and their diameters

Size	Diameter	Typical vehicle
205/55R16	24.9 in	Civic, Corolla, Golf
225/45R17	25.0 in	Accord, Camry, 3 Series
235/40R18	25.4 in	Plus-one fitment of the above
235/65R17	29.0 in	CR-V, RAV4, Escape
265/70R17	31.6 in	F-150, Silverado, Tacoma (“32s”)
275/55R20	31.9 in	Full-size trucks, 20-in wheels
285/70R17	32.7 in	Common off-road upgrade (“33s”)
35×12.50R17	35.0 in	Flotation size: diameter × width in inches

Before you change size

Keep the overall diameter within ±3 % unless you plan to recalibrate the speedometer. Check the load index and speed rating on the new size against the door-jamb placard, make sure the wheel width is within the tire’s approved rim range, and confirm clearance at full steering lock and full suspension compression. Wider tires with a lower profile grip better on dry pavement but ride harder and are more prone to rim damage from potholes.

Frequently asked questions

Cite this calculator

APACalcBrew. (2026). Tire Size Calculator. https://calcbrew.com/tire-size-calculator

MLA“Tire Size Calculator.” CalcBrew, 2026, calcbrew.com/tire-size-calculator.

Link to https://calcbrew.com/tire-size-calculator. Results are estimates; cite the underlying formula or data source for academic work.

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