Tire Size Guide Tire fitment math
Tire Size
Calculator
Compare two tire sizes and see exactly what changes: height, speedometer and clearance.
Last checked 12 October 2026Published by Sidewall GuideFormulas follow the Tire and Rim Association and ETRTO size conventions
Your result appears here as you type
Type your current tire size from the sidewall, for example 225/65R17.
A closer look
Small numbers, real changes
A tire’s outside diameter affects ride height and speedometer readings. Width changes how far the tread reaches on either side.
How plus-sizing worksCompare wheel offset and backspacing
Worked example
What changes from 225/65R17 to 235/60R18?
The new tire pairs a larger wheel with a shorter sidewall. Its calculated outside size is 0.59 in taller overall, a 2.1% increase. That means the car rises about half that amount, while the wider section moves each tread edge about 0.20 in on the same wheel offset.
| Measure | 225/65R17 | 235/60R18 | Change |
|---|---|---|---|
| Diameter | 28.52 in (724 mm) | 29.10 in (739 mm) | +0.59 in (+2.1%) |
| Section width | 8.86 in | 9.25 in | +0.39 in (+4.4%) |
| Sidewall | 5.76 in | 5.55 in | −0.21 in |
| Revs per mile | 707 | 693 | −14 |
| Actual speed at indicated 60 mph | 60.0 mph | 61.2 mph | +1.2 mph |
Check the change
Compare another pair
Use the calculator above with the sizes printed on your tires.
These are nominal size calculations; tire model, rim width, pressure and load can change measured dimensions. Check clearance and the vehicle maker’s load and fitment requirements before choosing a replacement.
About this calculator
Editorial checks
Written and checked by
Noam Cohen, editor of Tire Size Guide, built the calculator and checks its figures against the published formulas and cited references. Last checked 12 October 2026.
References and limits
Size conventions and tire guidance are informed by these published references:
- Tire and Rim Association — The Tire and Rim Association
- European Tyre and Rim Technical Organisation — ETRTO
- Tires — National Highway Traffic Safety Administration
- 49 CFR 571.110: Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information — eCFR (US Government)
- Tire markings explained — Michelin
We publish calculated size comparisons, not road tests or vehicle fitment approvals. No independent technical reviewer or consumer rating is represented here. See formulas, assumptions and all sources.
Learn more
Guides
What every part of 225/65R17 95H means, and where your size is printed.
Bigger wheels, lower sidewalls, the same overall height: plus one and plus two.
Why a taller tire makes the speedometer read low, and by how much.
What 95H means for the weight a tire carries and the speed it is built for.
Same dimensions, different construction and load rating: which suits a truck.
Inch sizes for lifted trucks, and how they convert to metric.
Common questions
Tire size questions
How do I read a tire size?
In 225/65R17, 225 is the width in millimeters, 65 is the sidewall height as a percentage of the width, R means radial construction, and 17 is the wheel diameter in inches. Any load index and speed symbol after the size describe capacity and speed rating. Read how to read a tire size
How much bigger can I go?
A diameter change within about 3% is a common rule of thumb, not a fitment guarantee. Check actual clearance at full steering lock and through suspension travel, and keep the replacement tire's load index at least as high as the vehicle requires. Wheel width and offset also affect fit. Read about plus-sizing and tire changes
Will bigger tires affect my speedometer?
Yes. A taller tire travels farther per revolution, so the speedometer reads low. For example, changing from 225/65R17 (28.52 in) to 235/60R18 (29.10 in) makes an indicated 60 mph about 61.2 mph in reality, roughly 2.1% faster. Read how tire size affects your speedometer
Do I need the same load index?
Meet or exceed the load index specified on your vehicle placard or in its owner's manual. A tire with a lower index may not safely carry the required load, even when its dimensions fit. For LT tires, compare the appropriate load range and pressure requirements too. Read about load index and speed rating
What is plus-sizing?
Plus-sizing means fitting a larger wheel with a shorter sidewall while keeping overall tire diameter close to the original. Plus one adds about an inch to wheel diameter; plus two adds about two inches. A close diameter helps preserve speedometer accuracy, but clearance, load rating, and approved wheel width still need checking. Read the plus-sizing guide
What size is a 33 inch tire in metric?
A common close metric equivalent is 285/75R16, which calculates to about 32.83 inches overall. The flotation size 33x12.50R15 is nominally 33 inches tall and 12.50 inches wide, on a 15-inch wheel. Actual dimensions vary by tire model. Read about flotation tire sizes
Calculation method
How this is calculated
Formulas
- Metric sidewall (mm) = width × aspect ratio ÷ 100.
- Overall diameter (in) = wheel diameter + 2 × sidewall (mm) ÷ 25.4.
- Section width (in) = width (mm) ÷ 25.4.
- Flotation sidewall (in) = (tire diameter − wheel diameter) ÷ 2; its diameter and width are the first two size numbers.
- Circumference (in) = π × diameter; revolutions per mile = 63,360 ÷ circumference.
- Millimetres = inches × 25.4. Percentage difference = (new − current) ÷ current × 100. Actual speed = indicated speed × new diameter ÷ current diameter.
Worked example: 225/65R17 → 235/60R18
- Current sidewall: 225 × 65 ÷ 100 = 146.25 mm, rounded to 146 mm = 5.76 in.
- Current diameter: 17 + 2 × 5.76 = 28.52 in (724 mm).
- New sidewall: 235 × 60 ÷ 100 = 141 mm = 5.55 in.
- New diameter: 18 + 2 × 5.55 = 29.10 in (739 mm).
- Section widths: 225 ÷ 25.4 = 8.86 in current; 235 ÷ 25.4 = 9.25 in new.
- Circumferences: π × diameter = 89.59 in current and 91.43 in new; 63,360 ÷ circumference = 707 and 693 revolutions per mile.
- Diameter change: (29.10 − 28.52) ÷ 28.52 = +2.1% (unrounded values are used for differences).
- At an indicated 60 mph: 60 × 29.10 ÷ 28.52 = 61.2 mph.
Rounding
Inches are rounded to 2 decimals, millimetres to whole numbers, revolutions per mile to whole numbers, percentages to 1 decimal with a sign, and speeds to 1 decimal. Differences are calculated from unrounded values before rounding.
Assumptions
- Sizes describe nominal, unloaded dimensions; actual dimensions vary by tire model and rim width.
- The comparison is a size calculation and does not confirm vehicle fitment or account for load, pressure, wear or measured tire growth.
Sources
The Tire and Rim Association
Michelin
Sidewall guide
How tire sizes work
- P means passenger tire.
- 225 is the section width in millimeters.
- 65 is the aspect ratio: sidewall height as a percentage of width.
- R means radial construction.
- 17 is the wheel diameter in inches.
- 95 is the load index.
- H is the speed rating.
Flotation sizes use a different order: 33x12.50R15 means 33 inches tall, 12.50 inches wide, on a 15-inch wheel.