Tire Size Calculator
Overall tire diameter = rim diameter + 2 × sidewall height. For 265/70R17: sidewall = 265 × 0.70 = 185.5 mm (7.30 in), so diameter = 17 + 2 × 7.30 = 31.6 in. Enter two sizes below to compare every dimension, see both tires to scale and the speedometer difference.
Compare tire sizes
The calculator needs JavaScript. The formulas and tables on this page work without it: sidewall = width × aspect ratio ÷ 100, and diameter = rim × 25.4 + 2 × sidewall (in mm).
Accepts 225/45R17, P225/45R17, LT285/75R16, 225/45ZR17 94W and 33x12.50R15.
Result
Next: speedometer error · diameter · plus sizing
How to use it
- Type your current size as A, for example 265/70R17.
- Type the new size as B (and C if you want a third).
- Read the differences, the drawing to scale and the speedometer table.
- Copy the link to share or save the comparison.
How the calculation works
| Value | Formula | Example: 265/70R17 |
|---|---|---|
| Sidewall height | width × aspect ratio ÷ 100 | 265 × 70 ÷ 100 = 185.5 mm (7.30 in) |
| Overall diameter | rim × 25.4 + 2 × sidewall | 17 × 25.4 + 2 × 185.5 = 802.8 mm (31.6 in) |
| Circumference | π × diameter | 3.1416 × 802.8 = 2522 mm (99.3 in) |
| Revolutions per mile | 63,360 ÷ circumference in inches | 63,360 ÷ 99.3 = 638 |
| Actual speed | speedometer × new diameter ÷ old diameter | 60 × 830.8 ÷ 802.8 = 62.1 mph on 285/70R17 |
These are nominal dimensions from the size code. Real tires differ slightly by make, model, tread depth and the rim they are fitted to.
Reading the result
- Diameter: the overall height of the tire. It sets your speedometer reading, gearing and ground clearance.
- Width: the section width, measured on the tire’s measuring rim. Wider tires need space inside the wheel arch and toward the suspension.
- Sidewall: the rubber between rim and tread. A lower sidewall usually means a firmer ride.
- Revolutions per mile or km: fewer revolutions means a taller tire.
Speedometer and the law
In countries that apply UN Regulation No. 39, paragraph 5.4 (EUR-Lex) (including the EU member states), a vehicle type is approved only if its speedometer never shows less than the true speed, and at the test speeds shows at most 10% plus 4 km/h more. That rule is about how new vehicles are approved. It does not say which tire sizes you may fit, and rules for changing tires differ between countries and states. Check locally, and ask a tire shop or the vehicle maker.
Before you change size
Rules about fitting a different tire size, vehicle inspection and insurance differ between countries and states, and can depend on what your vehicle’s papers list. This site does not give legal advice. Always check your vehicle owner’s manual, the tire placard in the door jamb, or a tire shop before changing size. Your inspection authority or insurer can tell you what applies to your vehicle.
Frequently asked questions
What does 265/70R17 mean?
265 is the section width in millimetres, 70 is the aspect ratio (the sidewall is 70% of the width, 185.5 mm), R means radial construction and 17 is the rim diameter in inches. The overall diameter works out at about 31.6 inches or 803 mm.
How much bigger is 285/70R17 than 265/70R17?
A 285/70R17 is about 831 mm (32.7 in) tall against 803 mm (31.6 in), so 3.5% larger in diameter. It is 20 mm wider and the ground clearance at the axle rises by about 14 mm.
Will bigger tires affect my speedometer?
Yes. A taller tire covers more ground per turn, so the speedometer shows less than your actual speed. With a 3.5% taller tire, 60 mph on the speedometer is about 62.1 mph. A dealer or a programmer can often recalibrate it.
How much bigger can I go?
That depends on your vehicle: wheel arch space, suspension, brakes, steering lock and what the vehicle’s papers allow. The colours in the calculator only show the size of the change. Check the owner’s manual, the door placard or a tire shop.
Why do real tires differ from the calculation?
The calculator uses the nominal dimensions in the size code. Real tires vary slightly between makes and models, tread depth changes the diameter as the tire wears, and the rim width changes the section width.
Can I share a comparison?
Yes. Copy link puts the sizes in the address, for example ?a=265-70R17&b=285-70R17, so anyone who opens it sees the same comparison. Nothing is stored on a server.
Last updated: October 15, 2026