What does a tire size like 265/70R17 mean?

265 is the section width in millimeters, 70 the aspect ratio (the sidewall is 70% of the width), R means radial and 17 is the wheel diameter in inches. A load index and speed symbol often follow, such as 113T. P in front marks a passenger tire and LT a light-truck tire.

What size is a 33 inch tire in metric?

None is exactly 33 in. Worked out from the codes, 285/75R16 is 32.83 in, 285/70R17 is 32.71 in and 255/80R17 is 33.06 in. Open Sizes near a diameter and enter 33 with your rim to list the nearest metric sizes, then check that a size is made.

How do I convert a metric tire size to inches?

Work out the overall diameter and the width in inches, and write them before the rim as a flotation size is written. For 285/75R16 the sidewall is 285 × 75% = 213.75 mm, twice that is 16.83 in, and with the 16 in rim the tire is 32.83 in tall; 285 mm is 11.22 in wide. So it is about 32.8x11.2R16. The line under each size box writes this for you, worked out from the code: a maker’s table can differ a little.

Is a 33x12.50R15 tire really 33 inches tall?

Not quite. The first number of a flotation size is a label: BFGoodrich lists its 33x12.50R15 Mud-Terrain at 32.5 in. The calculator works from the label unless you enter a measured diameter under Measured diameters.

How much lift do bigger tires give?

Half the change in diameter. 285/70R17 in place of 265/70R17 is 1.10 in taller, so the axles and everything they carry ride 0.55 in higher. The tire also grows 0.55 in toward the fender, so the gap above it closes by as much. A suspension lift is separate; the calculator shows the axle height change under the diameter.

Do bigger tires make the speedometer read faster or slower?

Slower. A taller tire covers more road each turn, so the dial shows less than your true speed: with 285/70R17 in place of 265/70R17, a dial showing 60 mph means about 62.1. A shorter tire makes it read high. The odometer reads low or high the same way.

Why does my speedometer not match the calculator or my GPS?

The calculator assumes the speedometer was right on the original size, and a speedometer can read a little off even on its original tires. Note a GPS speed and the dial’s at a steady speed on the original size, and enter both under My speedometer was already off: the table then starts from your own dial.

How do I correct the odometer, trip meter and fuel economy?

Multiply them by the distance factor. With 285/70R17 in place of 265/70R17 it is 1.0349: 100 miles on the trip meter is about 103.5 miles driven, and miles per gallon worked from it rise by the same factor. Divide L/100 km by it.

What number do I enter to recalibrate the speedometer?

It depends on the tool or the shop: some ask for the new tire’s diameter, some its circumference, some its revolutions per mile. The calculator gives all three in one block to copy. Give the original’s on the same basis, both worked out from the code or both measured.

A larger wheel with a lower-profile tire that keeps the overall diameter: 225/65R17 to 225/60R18 is 0.40% taller, so the speedometer barely moves, while the sidewall drops by 11.3 mm. Plus one is an inch of wheel, plus two two inches.

Why are the revolutions per mile different from the tire maker’s?

The calculator works them out from the size code’s diameter, and the makers’ own tables run about 3% higher for every size checked. Michelin lists 730 revolutions per mile for 225/65R17 against 707.3 here. The ratio between two sizes, which the speedometer follows, agrees.

Do worn tires change the speedometer reading?

Yes, a little. A tire worn from 10.5/32 to 2/32 of an inch has lost about 0.53 in of diameter, so a 225/65R17 is 1.86% shorter and a dial showing 60 mph means about 58.9. Enter the tread depths under Worn tires; the result is an estimate.

Can I use a lower load index or speed rating than my original tires?

Michelin says never to choose a tire with less load-carrying capacity than the one the vehicle came with, and that a lower speed rating lowers the vehicle’s top speed limit to the tire’s. Add the load index and speed symbol to both sizes: the calculator flags the new one when it is lower.

Does this calculator check whether a new size clears my vehicle?

No. It compares the two sizes as their codes describe them. Clearance at the fenders, suspension and brakes, rim width and offset, the vehicle’s load and the sizes the vehicle maker lists are not checked. The placard on the driver’s door edge and the owner’s manual give the original size.

Tire Size Calculator - Compare Sizes & Speedometer Error

Formulas and sources

Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.

Sidewall and overall diameter

For a metric size

Sidewall = Width × Aspect ÷ 100 Diameter = Rim × 25.4 + 2 × Sidewall

Width: the section width, in mm
Aspect: the aspect ratio: the sidewall as a percentage of the width
Rim: the wheel diameter, in inches
Sidewall: the sidewall height, in mm
Diameter: the overall diameter, in mm

P, LT, ST, T and Euro-metric sizes are worked the same way, and R, ZR, D or B changes nothing in the arithmetic. A load index and speed symbol after the size are read for the load and speed only. In inches the size is written as a flotation size is, Diameter x Width then the rim, each to a tenth: 265/70R17 is about 31.6x10.4R17.

Source: The parts of a size as Michelin defines them [1]. The formula is written out as D = d + 2H, with H the width × 0.01 × the aspect ratio, in AIS-044 (Part 2), clause 6.1.2.2.1 [2], which lets a new tire’s sidewall term run 0.97 to 1.04 times it.

A flotation size

For a flotation size

Sidewall = (Diameter − Rim) ÷ 2

Diameter: the first number, the overall diameter in inches
Rim: the wheel diameter, in inches
Sidewall: the sidewall height, in inches

In 33x12.50R15, 33 is the overall diameter, 12.50 the section width and 15 the rim, all in inches. The first number is a label: the maker’s table lists that tire at 32.5 in.

Source: Bridgestone’s high flotation sizing system [3]; BFGoodrich’s Mud-Terrain T/A KM3 size table [6].

Circumference and revolutions

Circumference = π × Diameter Revs per mile = 63,360 ÷ Circumference (in) Revs per km = 1,000,000 ÷ Circumference (mm)

Circumference: once round the tire
Diameter: the overall diameter in use
Revs per mile: turns of the tire in a mile
Revs per km: turns of the tire in a kilometer

A maker’s table gives the real tire: close to these diameters, within about 0.5% for metric sizes and 2% for revolutions per km, but its revolutions per mile run about 3% higher. The ratio between two sizes agrees.

Source: Standard formula; the inch is 25.4 mm and the mile 63,360 in, exactly [4]. Compared with the size tables of Michelin [5], BFGoodrich [6] [7] and Bridgestone [8] [9].

The change

With a new size

Change % = 100 × (New − Original) ÷ Original

New: the new size’s figure
Original: the original’s figure

For every row of the table: diameter, width, sidewall, circumference and revolutions.

Source: Standard formula; no single publisher.

The speedometer

With a new size

Factor = New diameter ÷ Original diameter True speed = Dial speed × Factor Dial speed = True speed ÷ Factor

Factor: the true speed ÷ the dial speed
New diameter: the new size’s overall diameter
Original diameter: the original’s overall diameter
True speed: the speed over the road
Dial speed: what the speedometer shows

It assumes the speedometer was right on the original size. A taller tire makes the factor above 1, and the dial reads low; a shorter one, below 1, and it reads high.

Source: Discount Tire: a taller tire gives a reading that “displays slower than your actual speed”, a shorter one a faster speed [10]. The factor is the ratio of the two diameters, as the dial counts wheel turns.

A dial that was already off

With My speedometer was already off

Factor = New diameter ÷ Original diameter × GPS ÷ Dial

GPS: a GPS or radar speed, on the original size
Dial: what the dial showed at that moment

The dial’s own error on the original size carries over, so the table starts from your speedometer rather than a perfect one. It is not applied to the distance factor.

Source: CalcTrail’s own correction from your two readings; no single publisher.

The odometer and fuel economy

With a new size

True distance = Distance shown × New diameter ÷ Original diameter

True distance: the distance over the road
Distance shown: the odometer’s or trip meter’s figure

Miles per gallon or km per liter worked from the trip meter take the same factor; L/100 km is divided by it. It assumes the odometer was right on the original size.

Source: Standard formula; no single publisher.

The axle height

With a new size

Axle height change = (New diameter − Original diameter) ÷ 2

Axle height change: how much higher the axle rides, or lower
New diameter: the new size’s overall diameter
Original diameter: the original’s overall diameter

The axle sits half the tire’s diameter above the road, so it moves half the change, and the top of the tire moves the same toward the fender. A suspension lift is separate.

Source: Standard geometry, the radius as half the diameter; no single publisher.

Worn tires

With Worn tires

Worn diameter = Diameter − 2 × (Tread new − Tread now)

Worn diameter: the New size’s diameter, worn
Tread new: the tread depth when new
Tread now: the tread depth left

An estimate for the New size, skipped when it has a measured diameter, which already holds the wear. Tread in 32nds of an inch or in mm.

Source: No single publisher states it; it follows from tread depth, measured on each side of the tire. NHTSA gives 2/32 of an inch as the depth to replace a tire at [15].

Measured diameters

A diameter you measure replaces the worked-out one for the diameter, the circumference, the revolutions and the speedometer. The width and the sidewall stay the size code’s.

Source: Michelin’s size figures are “average design values”, and a tire “may vary from this value by +/-3% of the section height” [5].

The band

The change in overall diameter is read against the band you set, 3% unless you change it, and the band is named beside every inside or outside. It is a guideline, not a limit.

Source: Les Schwab: “within 3 percent of the diameter (height) measurement of the tires originally on your vehicle” [11]. No tire maker or standards body sets a figure.

Sizes near a diameter

Widths 145 to 355 mm in steps of 10 and aspect ratios 25 to 85 in steps of 5, on the rim you choose, worked out with the formula above and listed nearest first, at most 12. They are not a list of sizes on sale.

Source: The diameter formula of AIS-044 (Part 2) [2].

Load index and speed symbol

Load index 70 to 130 in kilograms, with pounds converted from them and rounded, so a maker’s table may print a pound less. Speed symbols L to Y, H between U and V; Z and (Y) are above 240 and 300 km/h. A light-truck pair is the load for single and dual wheels. A new tire lower on either is flagged.

Source: Kilograms: AIS-044 (Part 2) Annexure B for 70 to 120 [2] and Kleber’s table for 121 to 130 [12]; pounds with NIST’s exact pound [4]. Speeds: AIS-044 clause 2.29 [2], in mph as Michelin’s chart prints them [13]. Single and dual: Yokohama [14]. Michelin: never less load-carrying capacity than the original [13]; NHTSA: the original size or another the vehicle maker lists [15].

Sources

Michelin North America. Tire Markings Explained: How to Read a Tire, the tire size: section width, aspect ratio, construction and wheel diameter; prefixes. Updated October 7, 2026; read October 16, 2026.
Automotive Industry Standards Committee, Government of India (ARAI). AIS-044 (Part 2): Automotive Vehicles - Pneumatic Tyres for Passenger Car Vehicles, clauses 2.29 and 6.1.2.2; Annexures A, B and C. With Amendments 1 and 2; read October 16, 2026.Our automated tests check this calculator against 70 cases from it.
Bridgestone Americas Tire Operations. Tire Terminology, high flotation sizing system; P-metric and LT-metric; plus 1 and plus 2; revolutions per mile. No page date; read October 16, 2026.
National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science, length, the inch and the mile; mass, the pound. Special Publication 811.
Michelin North America. MICHELIN Defender2: size specifications, the size table and the notes under it. Size data dated October 10, 2026; read October 16, 2026.Our automated tests check this calculator against 4 cases from it.
BFGoodrich Tires (Michelin North America). BFGoodrich Mud-Terrain T/A KM3: size specifications, the size table: overall diameter, section width, revolutions per mile and per km. Read October 16, 2026.Our automated tests check this calculator against 3 cases from it.
BFGoodrich Tires (Michelin North America). BFGoodrich All-Terrain T/A KO2: size specifications, the size table: overall diameter, section width, revolutions per mile and per km. Read October 16, 2026.Our automated tests check this calculator against 1 case from it.
Bridgestone Americas Tire Operations. Bridgestone Dueler H/T 685: size specifications, the size table: overall diameter, revolutions per mile and per km. Read October 16, 2026.Our automated tests check this calculator against 2 cases from it.
Bridgestone Americas Tire Operations. Bridgestone Turanza T005: size specifications, the size table: overall diameter, revolutions per mile and per km. Read October 16, 2026.Our automated tests check this calculator against 1 case from it.
Discount Tire. Tire Size and Speedometer Accuracy, up-sizing and downsizing; recalibration. No page date; read October 16, 2026.Our automated tests check this calculator against 2 cases from it.
Les Schwab Tire Centers. Tire Size Calculator: What Size Tires Should You Get?, the 3 percent general rule. Published December 20, 2022; read October 16, 2026.
Kleber (Michelin group). Load ratings, the load index table (kg). No page date; read October 16, 2026.Our automated tests check this calculator against 10 cases from it.
Michelin North America. Changing Tire Size or Specifications, the speed rating chart; the size, load capacity and speed rating answers. Updated June 8, 2026; read October 16, 2026.Our automated tests check this calculator against 14 cases from it.
Yokohama Tire Corporation. How to Read a Sidewall: Sizing Information, service descriptions; light-truck single and dual load indices. No page date; read October 16, 2026.Our automated tests check this calculator against 2 cases from it.
National Highway Traffic Safety Administration. Tires, tire size; tire tread. Read October 16, 2026.

What this calculator does not cover

It works from the size code, or the diameter you measure; it has no table of real tires and does not say whether a size is made.
It does not check clearance at the fenders, suspension or brakes, rim width, wheel offset or the sizes the vehicle maker lists.
It does not check the vehicle’s load against the tire’s, or the lower load a tire carries above 210 km/h.
It does not read numeric light-truck sizes such as 7.50R16, which carry no aspect ratio, millimeter rims or bicycle sizes.
It says nothing on local rules or inspections, ABS, traction and stability control, or how to recalibrate a speedometer.

Sources checked on October 16, 2026.

Help

Type the original size: Enter the size the speedometer was set for, as printed on the sidewall or the placard on the driver’s door edge, such as 265/70R17 or P225/65R17 102H.
Type the new size: Enter the size to compare, such as 285/70R17 or 33x12.50R15LT. Leave it blank to read the original alone.
Read the comparison: The table gives each figure in inches and millimeters, with the change and the percent, and the change in diameter against your band.
Read the speedometer: Pick mph or km/h. The table gives the true speed at each dial speed, and what the dial shows at each true speed, with the distance factor for the odometer.
Add what you know: Open Measured diameters for a tape measure’s figure, My speedometer was already off for a GPS check, or Worn tires for the tread left.
Find sizes near a diameter: Open Sizes near a diameter, enter a target and a rim, and press Compare on a size to set it as the new one.

Understanding Tire Sizes

Reading a tire size

In 265/70R17, 265 is the section width in millimeters, 70 is the aspect ratio, the sidewall as a percentage of the width, R means radial, and 17 is the wheel diameter in inches. A P in front marks a passenger tire, LT a light-truck tire, and no letter the Euro-metric form, worked the same way. A flotation size such as 33x12.50R15 gives the overall diameter and the width in inches. The line under each box says what the code means.

Diameter, sidewall and revolutions

The sidewall is the width times the aspect ratio: 265 × 70% is 185.5 mm. The overall diameter is the rim, 17 in or 431.8 mm, plus two sidewalls: 802.8 mm, or 31.61 in. Once round the tire is π times that, 99.29 in, so it turns 638.1 times in a mile and 396.5 times in a kilometer.

The speedometer and the odometer

A speedometer counts wheel turns. A taller tire covers more road each turn, so the dial reads low: going from 265/70R17 to 285/70R17 makes the tire 3.49% taller, and a dial showing 60 mph means about 62.1. At a true 60 the dial shows 58.0. The odometer and trip meter read low the same way: multiply a distance shown by 1.0349 for the true one.

Worked out against measured

The figures come from the size code. Makers’ tables give a real tire close to them in diameter, but their revolutions per mile run about 3% higher, and a flotation label is not a measurement: BFGoodrich lists its 33x12.50R15 at 32.5 in. The change between two sizes, which the speedometer follows, holds either way. A measured diameter replaces the worked-out one.

The 3% guideline

Some tire shops suggest keeping a new size within 3% of the original’s overall diameter; Les Schwab calls it a general rule. No tire maker or standards body sets that figure, and it is not a limit. The calculator reads the change against the band you set, 3% unless you change it, and names the band with every result.

Plus sizing and winter wheels

Plus one keeps the diameter on a rim an inch larger with a lower sidewall: 225/65R17 to 225/60R18 is 0.40% taller, while the sidewall drops from 146.3 to 135.0 mm. A winter set on a smaller wheel goes the other way: 225/45R17 to 205/55R16 is 0.38% shorter, with a taller sidewall. Sizes near a diameter lists metric sizes close to a target on any rim.

Load index and speed symbol

The number and letter after a size, such as 102H, are its service description: load index 102 carries 850 kg (1,874 lb) per tire, and speed symbol H is 210 km/h (130 mph). The letters are not in alphabetical order: H sits between U and V. A light-truck pair such as 121/118 gives the load for single and dual wheels. The calculator flags a new tire that is lower than the original on either.

Tire Size Calculator - Compare Sizes & Speedometer Error

Formulas and sources

Sidewall and overall diameter

A flotation size

Circumference and revolutions

The change

The speedometer

A dial that was already off

The odometer and fuel economy

The axle height

Worn tires

Measured diameters

The band

Sizes near a diameter

Load index and speed symbol

Sources

What this calculator does not cover

Help

Understanding Tire Sizes

Reading a tire size

Diameter, sidewall and revolutions

The speedometer and the odometer

Worked out against measured

The 3% guideline

Plus sizing and winter wheels

Load index and speed symbol

Bigger Tires After a Lift

What Is a 33 in Metric?

Plus One, Same Diameter

Winter Set on Smaller Wheels

Small Car Upsize in km/h

Worn Tires Against New

A Dial That Read High

Light-Truck Load and Speed

A Measured Flotation Tire

Frequently Asked Questions

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Unit Converter

Percentage Calculator

Tire Size Calculator - Compare Sizes & Speedometer Error

Formulas and sources

Sidewall and overall diameter

A flotation size

Circumference and revolutions

The change

The speedometer

A dial that was already off

The odometer and fuel economy

The axle height

Worn tires

Measured diameters

The band

Sizes near a diameter

Load index and speed symbol

Sources

What this calculator does not cover

Help

Quick Start Guide

Understanding Tire Sizes

Reading a tire size

Diameter, sidewall and revolutions

The speedometer and the odometer

Worked out against measured

The 3% guideline

Plus sizing and winter wheels

Load index and speed symbol

Features

Common Use Cases

Examples

Bigger Tires After a Lift

What Is a 33 in Metric?

Plus One, Same Diameter

Winter Set on Smaller Wheels

Small Car Upsize in km/h

Worn Tires Against New

A Dial That Read High

Light-Truck Load and Speed

A Measured Flotation Tire

Frequently Asked Questions

Related Calculators

Fuel Cost Calculator

Unit Converter

Percentage Calculator