How do you calculate tire diameter from the size?

Multiply the section width by the aspect ratio to get the sidewall height, double it, and add the wheel diameter converted to millimeters. For 265/70R17: 265 × 0.70 = 185.5 mm of sidewall, and (17 × 25.4) + (2 × 185.5) = 802.8 mm, which is 31.61 inches.

What does the 70 in 265/70R17 mean?

It is the aspect ratio: the sidewall height as a percentage of the section width. It is not a measurement in its own right, which is why two tires with the same 70 can have very different sidewalls — 265/70R17 has a 185.5 mm sidewall, 315/70R17 has 220.5 mm.

How much can I change tire size without a problem?

The industry rule of thumb is to stay within 3% of the original overall diameter. Beyond that the speedometer, odometer and ABS calibration all drift by the same percentage, because every one of them is derived from revolutions per mile.

Why does my tire's spec sheet show more revs per mile than this calculator?

Manufacturers measure revolutions per mile on a loaded tire, whose rolling circumference is about 3% shorter than the free diameter. This calculator shows the geometric figure — the one speedometer maths uses — and the loaded estimate alongside it.

Can this tell me what tires fit my truck?

No, and nothing that only knows the size code can. Fitment depends on suspension travel, rim offset and body clearances that live in licensed manufacturer data. This site tells you exactly what a size measures and what changes if you fit a different one.

Tire size arithmetic

Tire Size Calculator

Enter a tire size and this works out its overall diameter, sidewall height, rolling circumference and revolutions per mile. A 265/70R17 stands 31.61″ tall, because its sidewall is 265 × 0.70 = 185.5 mm and the wheel adds another 431.8 mm. No sign-up, no vehicle lookup, and every step of the arithmetic is shown.

Tire size

Metric (265/70R17), with a prefix (LT285/70R17) or flotation (33x12.50R15).

Common sizes

Overall diameter

31.61″

802.8 mm · 31 5/8″ on a tape

Sidewall

7.30″

185.5 mm

Section width

10.43″

265 mm

Revs / mile

638.1

396.5 / km

265/70R17 31.61″

Full dimensions

Derived dimensions for 265/70R17
Dimension	Metric	Imperial
Section width	265.0 mm	10.43″
Sidewall height	185.5 mm	7.30″
Wheel diameter	431.8 mm	17.0″
Overall diameter	802.8 mm	31.61″
Rolling circumference	2,522.1 mm	99.29″
Revolutions per mile	396.5 / km	638.1 / mile
Measuring rim (estimate)	203 mm	8″

The arithmetic

1Sidewall = 265 mm × 70 ÷ 100 = 185.50 mm
2Wheel = 17.0″ × 25.4 = 431.8 mm
3Overall diameter = 431.8 + (2 × 185.50) = 802.8 mm = 31.61″
4Circumference = π × 31.61″ = 99.29″
5Revolutions per mile = 63,360 ÷ 99.29 = 638.1

A tire maker's specification sheet would print roughly 658 revolutions per mile for this size rather than 638, because that figure is measured under load, where the rolling circumference is about 3% shorter. Speedometer maths uses the geometric figure shown here.

Compare this size against another →

Recommended rim width for this size is about 7.5″ to 9″, measured on a 8″ rim. How rim width is derived

How is tire size calculated?

A metric tire code is three measurements and a construction letter. In 265/70R17 the 265 is the section width in millimeters, the 70 is the aspect ratio as a percentage of that width, R means radial construction, and the 17 is the wheel diameter in inches. Only two of the four are lengths, which is why the code cannot be read at a glance.

The sidewall is the number the code hides. It is section width × aspect ratio ÷ 100 — for 265/70R17, 185.5 mm. Overall diameter is the wheel plus two sidewalls, one above the hub and one below: (17 × 25.4) + (2 × 185.5) = 802.8 mm, or 31.61 inches. Everything else follows from that single figure. Circumference is π × diameter, and revolutions per mile is 63,360 ÷ circumference in inches, because there are 63,360 inches in a mile.

The 25.4 is not an approximation. The inch has been defined as exactly 25.4 millimeters since the international yard and pound agreement of 1959, so a metric tire code and an inch wheel diameter combine without any rounding error at all.

Why does tire size change the speedometer?

A speedometer does not measure road speed. It counts wheel revolutions and multiplies by a constant that was set at the factory for the original tire's revolutions per mile. Fit a taller tire and each revolution covers more ground, so the same count means you have travelled further than the dash believes.

The error is exactly proportional to the diameter change. Go from 265/70R17 (31.61″) to 285/70R17 (32.71″) — a 3.49% increase — and an indicated 60 mph is really 62.1 mph. The odometer under-counts by the same 3.49%, which over 10,000 indicated miles is about 349 miles you drove but were never charged for in a lease or warranty. The speedometer error calculator works this out for any pair of sizes.

What is the 3% rule?

Tire fitters use ±3% of original overall diameter as the boundary for a safe substitution. Inside it, the speedometer error stays within the tolerance most jurisdictions allow, ABS and traction control keep working from wheel-speed ratios that still make sense, and the gearing feels unchanged.

It is a rule of thumb, not a regulation, and it says nothing about whether the tire physically fits. A size can be 0.2% off in diameter and still foul the strut because it is 20 mm wider. Diameter and clearance are separate questions.

205/55R16 vs 225/45R1724.88″ against 24.97″265/70R17 vs 285/70R1731.61″ against 32.71″265/70R17 vs 33x12.50R1731.61″ against 33.00″

Which sizes get compared most

Every comparison below is a permanent page with the full arithmetic worked out — there are 557 of them, built from 108 sizes that are actually fitted as original equipment or sold as common upgrades.

Browse all 557 size comparisons →

Common questions about the tire size calculator

How do you calculate tire diameter from the size?: Multiply the section width by the aspect ratio to get the sidewall height, double it, and add the wheel diameter converted to millimeters. For 265/70R17: 265 × 0.70 = 185.5 mm of sidewall, and (17 × 25.4) + (2 × 185.5) = 802.8 mm, which is 31.61 inches.
What does the 70 in 265/70R17 mean?: It is the aspect ratio: the sidewall height as a percentage of the section width. It is not a measurement in its own right, which is why two tires with the same 70 can have very different sidewalls — 265/70R17 has a 185.5 mm sidewall, 315/70R17 has 220.5 mm.
How much can I change tire size without a problem?: The industry rule of thumb is to stay within 3% of the original overall diameter. Beyond that the speedometer, odometer and ABS calibration all drift by the same percentage, because every one of them is derived from revolutions per mile.
Why does my tire's spec sheet show more revs per mile than this calculator?: Manufacturers measure revolutions per mile on a loaded tire, whose rolling circumference is about 3% shorter than the free diameter. This calculator shows the geometric figure — the one speedometer maths uses — and the loaded estimate alongside it.
Can this tell me what tires fit my truck?: No, and nothing that only knows the size code can. Fitment depends on suspension travel, rim offset and body clearances that live in licensed manufacturer data. This site tells you exactly what a size measures and what changes if you fit a different one.