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Will the new tires actually fit?
Enter two sizes. Get the diameter change, speedometer error and a straight answer — in about ten seconds.
Tire size calculator
Pick two common sizes or type any custom dimensions. Results update as you type.
205/55R16
225/45R17
225/45R17 on a vehicle fitted with 205/55R16: Good fit
Within the 3% rule — overall diameter changes only 0.4%. Speedometer calibration, ABS, traction control and gearing all stay in their intended range. Your speedometer will read lower than your true speed.
| Measurement | 205/55R16 | 225/45R17 | Change |
|---|---|---|---|
| Overall diameter | 24.88″ | 24.97″ | +0.09″ (+0.4%) |
| Sidewall height | 4.44″ (112.8 mm) | 3.99″ (101.3 mm) | −11.5 mm |
| Tread width | 205 mm | 225 mm | +20 mm |
| Circumference | 78.16″ | 78.45″ | +0.30″ |
| Revs per mile | 811 | 808 | −3 |
| Speedometer error | baseline | +0.4% | reads low |
When your speedometer shows the indicated speed with 225/45R17 fitted, your actual speed is:
25 mph shown
25.1 mph actual
35 mph shown
35.1 mph actual
45 mph shown
45.2 mph actual
55 mph shown
55.2 mph actual
65 mph shown
65.2 mph actual
75 mph shown
75.3 mph actual
The 3% & 5% diameter rules
Tire professionals judge a swap by how much the overall diameter changes. That single percentage drives everything else: speedometer accuracy, clearance, gearing and electronic safety systems.
Green zone
The industry-standard safe range. Diameter stays close enough that your speedometer, odometer, ABS and stability control keep working as calibrated. Most plus-size upgrades live here.
Yellow zone
Workable, but do your homework. Expect measurable speedometer error and check wheel-well clearance at full lock and full compression, plus scrub radius and gearing effects.
Red zone
Avoid. Large diameter changes throw off speedometer and transmission calibration, strain driveline components, and risk rubbing. Recalibration is usually required to drive safely.
How to read a tire size
Find the code on your tire’s sidewall — for example 225/45R17. Each part tells the calculator what it needs.
225/45R17
225
Tread width
The tire’s nominal section width, in millimeters, measured sidewall to sidewall.
45
Aspect ratio
Sidewall height as a percentage of width. Here: 45% of 225 mm ≈ 101 mm of sidewall.
R17
Construction & wheel
R means radial construction; 17 is the wheel diameter in inches the tire mounts on.
Popular size-vs-size comparisons
Pre-computed comparison pages for the swaps drivers check most.
Not sure which size you have?
Browse a library of common tire sizes by wheel diameter — with overall diameter, circumference and revs per mile for every size, plus one-click comparisons.
Open the tire size libraryTire size questions, answered
- How much bigger can my tires be?
- Keep the new overall diameter within 3% of the original and everything calibrated to the factory size — speedometer, odometer, ABS and stability control — keeps working as intended. Between 3% and 5% is usually workable but needs a clearance check and leaves a measurable speedometer error. Beyond 5% you are into recalibration territory and real rubbing risk.
- Does changing tire size affect the speedometer?
- Yes, and the error is proportional to the diameter change. A tire 3% larger turns 3% fewer times per mile, so at an indicated 60 mph you are actually doing about 61.8 mph. Larger tires make the speedometer read low; smaller tires make it read high.
- How do I read a tire size like 225/45R17?
- 225 is the tread width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of that width, so 45% of 225 mm is about 101 mm. R means radial construction, and 17 is the wheel diameter in inches.
- Can I fit wider tires on the same wheels?
- Often yes, within the wheel rated width range, but width and diameter move together. Going wider while keeping the same aspect ratio also makes the tire taller, so check the diameter change here first and confirm inner-fender and strut clearance before buying.
How these numbers are calculated
Every figure on this site is derived from the tire size code itself — there is no lookup table and no estimation. Sidewall height is tread width × aspect ratio ÷ 100. Overall diameter is the wheel diameter plus two sidewalls. Circumference is π × diameter, and revolutions per mile is 63,360 ÷ circumference. Speedometer error is simply the ratio of the two overall diameters, because a speedometer counts wheel rotations and assumes the factory rolling circumference.
These are nominal dimensions from the size code. Real molded dimensions vary by a few millimetres between manufacturers and models, and a mounted tire’s loaded radius is smaller than its free radius. Treat the results as accurate to within roughly 1% and confirm fitment against your vehicle’s tire and loading information placard.
Sources
- 49 CFR § 571.110 — Tire selection and rims, the federal standard behind the tire and loading information placard on your driver’s door jamb.
- NHTSA TireWise — Tire safety ratings and awareness, for tire maintenance, tread depth and load-rating guidance.
Last updated September 2026.