Tire Size Calculator

Getting a number you can act on

1. 01
   Read the size off the sidewall, not the invoice

   The size is moulded into the rubber in the form 225/65R17. What is fitted now is not always what the vehicle left the factory with, so take the number from the tire in front of you.

2. 02
   Type it in any format you like

   225/65R17, 225 65 17, P225/65R17 and LT225/65R17 all resolve to the same geometry. The service prefix describes the tire's intended duty, not its dimensions.

3. 03
   Read the overall diameter first

   This is the number that matters most. It sets ride height, gearing, speedometer accuracy and whether the tire clears the arch at full lock.

4. 04
   Check revolutions per mile against your current size

   The speedometer and odometer are both calibrated to a specific revs-per-mile figure. Change it and both readings shift by the same proportion.

5. 05
   Compare before you commit

   If you are changing size rather than replacing like for like, run both sizes through the comparison tool. A size that looks close on paper can be an inch taller in practice.

What the calculator is actually doing

Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.

Why the aspect ratio catches people out

The first number in a metric tire size is a width in millimetres. The last number is a rim diameter in inches. The middle number is neither — it is a percentage.

That mixed notation is the source of most sizing mistakes. A 55-series tire does not have a 55 mm sidewall; it has a sidewall equal to 55% of its section width. So 225/55R17 has a 123.75 mm sidewall, while 275/55R17 has a 151.25 mm one. Same aspect ratio, sidewalls nearly 28 mm apart, and an overall diameter difference of well over two inches.

This is also why widening a tire without lowering the aspect ratio makes it taller. If you go from 225/55R17 to 245/55R17, you have not just added 20 mm of width — you have added 11 mm of sidewall at each end and roughly 0.87 inches of overall diameter. That is often the difference between clearing an inner liner and rubbing on it.

Flotation sizes work the other way round

Light-truck tires are frequently sold in flotation sizing instead: 33x12.50R15. Here the notation is far more direct, because every figure is already in inches.

The first number is the overall diameter — 33 inches. The second is the section width — 12.5 inches. The last is the rim diameter, 15 inches. There is no aspect ratio to calculate, because the diameter is stated outright.

That makes flotation sizes easier to reason about but harder to compare with metric ones, which is exactly what this calculator is for. It converts both notations into the same set of measurements so you can put a 285/70R17 and a 33x12.50R17 next to each other honestly. In that particular case they are within a quarter of an inch of each other, which surprises a lot of people.

Who decides what a tire size means

The size on a sidewall is not a manufacturer's description. It is a standardised designation, and there are bodies whose job is to keep it consistent.

In the United States that body is the Tire and Rim Association, which has been the standardising authority for tire and rim interchangeability since 1903. Europe has ETRTO, and ISO 5775 covers the international designation system. Between them they define what each figure in a size means and, importantly, which rim widths a given section width may legitimately be mounted on.

That last part matters more than most people realise. Every section width has an approved rim-width range rather than a single correct wheel. As a rough guide, the ideal rim inner width is around 80 to 90 percent of the section width converted to inches, with an approved band of roughly an inch either side of that ideal.

So a 245-section tire — 9.65 inches wide — sits happiest on something near 8 inches, and the approved range typically spans about 7 to 9. Mount it outside that band and the sidewall is pulled into a shape it was not designed to hold, which changes the contact patch, the handling and the wear pattern regardless of what the size calculation says.

What overall diameter actually controls

• Ride height — half the diameter change appears at the axle, so a 1-inch taller tire lifts the vehicle half an inch
• Effective gearing — a taller tire acts like a numerically lower axle ratio, dulling acceleration and lowering cruising RPM
• Speedometer and odometer accuracy, both of which shift by the same percentage as the diameter
• Arch and suspension clearance, particularly at full steering lock and full suspension compression
• Anti-lock braking and stability control, which infer wheel speed from a calibrated rolling radius
• Load rating and pressure requirements, which change with the air volume the tire encloses

The 3% rule, and where it comes from

The convention across the industry is that a replacement size should stay within about 3% of the original overall diameter. It is not a legal limit in most places, and it is not a hard engineering boundary — it is a practical tolerance.

It exists because 3% is roughly where several things start to matter at once. Speedometer error becomes noticeable rather than academic. Clearance margins designed around the original size start being consumed. And systems that infer vehicle speed from wheel rotation begin working with an assumption that is measurably wrong.

Inside 3% you are generally making a change the vehicle can absorb. Outside it, you are making a modification, and the honest approach is to treat it as one — check clearance physically, expect the speedometer to be off, and consider whether recalibration or a gearing change is warranted.