Tire size comparison calculator

What the tire numbers mean

In 225/45R17, 225 is nominal section width in millimeters, 45 is sidewall height as a percentage of that width, R means radial construction and 17 is wheel diameter in inches. The wheel diameter must match the tire. These dimensions do not include its load index or speed rating. Michelin explains tire size markings and the vehicle placard.

Find the original size on the vehicle's tire placard or in its manual. On a used car, the tires currently installed may differ from the approved specification. If front and rear sizes differ, compare each axle separately and check the permitted combination.

How the calculator works

The calculation uses the printed dimensions, with 25.4 millimeters per inch:

• Sidewall height = width × aspect ratio ÷ 100.
• Overall diameter = wheel diameter × 25.4 + twice the sidewall height.
• Diameter change = (new diameter ÷ original diameter − 1) × 100.
• Estimated road speed = indicated speed × new diameter ÷ original diameter.

The speed ratio is the same whether you enter mph or km/h. The unit selector labels your input and result; it does not convert the number you entered. Radius change is half the diameter change and describes unloaded geometry, not a measured ride-height increase.

Example: 225/45R17 versus 235/40R18

The original diameter is 634.3 mm; the proposed size is 645.2 mm. That is about 1.72% larger, with a 10 mm increase in nominal width and a shorter sidewall. At an indicated 60 mph, the simple diameter model gives about 61.0 mph.

This example also needs an 18-inch wheel in place of the original 17-inch wheel. The result cannot tell you whether that wheel's width, offset, bolt pattern or brake clearance suits a particular car. Compare those separately in our wheel offset and fitment guide.

Try reversing the two inputs. The estimated speed should move below the indicated speed when the proposed diameter is smaller. Entering identical sizes should produce zero dimensional change and the same estimated and indicated speed.

Is a tire within 3% automatically safe?

No. This calculator deliberately does not label a percentage difference “safe” or “approved.” NHTSA recommends the original size or another size recommended by the vehicle manufacturer. A generic percentage rule does not replace that recommendation.

Michelin's plus-sizing guidance calls for checking vehicle-manufacturer restrictions. Increasing wheel diameter generally requires a lower tire aspect ratio to keep overall diameter similar, but geometric similarity is only one part of the choice.

P and LT prefixes do not change this dimensional formula. They do change the specification you need to assess: matching the displayed numbers does not make passenger and light-truck tires interchangeable.

What to verify before ordering

1. Ask whether the proposed size and axle combination are approved for the exact vehicle and trim.
2. Check the tire maker's approved rim-width range and the tire's actual published dimensions.
3. Confirm load capacity, speed rating, tire type and the correct inflation requirements for the application.
4. Have the installer check clearance through steering travel and suspension movement, including the brakes.
5. Ask about speedometer calibration, tire-pressure monitoring and any drivetrain restrictions, particularly when mixing front and rear sizes.

Bring the original specification, proposed tire's product sheet and wheel specification to the installer. Saving only a screenshot of the percentage difference leaves most of the fitment decision unanswered.

Choose the tire's job as well as its size

Once the fitment is confirmed, compare the tire for the roads and climate where you drive. Our all-season, all-weather and winter comparison covers seasonal selection. For a performance-car purchase, the Michelin and Continental comparison addresses a separate product choice.

This tool does not recommend inflation pressures. Use the vehicle's approved specification and address warning lights with the tire-pressure troubleshooting flowchart. If you are budgeting for wheels and tires together, include fitting and related work in the modification budget planner.

Calculation scope

Published October 10, 2026. Results use nominal geometry and omit tread wear, tire deflection, construction differences and existing speedometer error. The input range is a calculator limit, not a catalog of available or approved sizes. This is a comparison aid; it cannot approve a tire installation.