Will the new tires actually fit?
Put in the size you have and the size you are thinking about. You get the diameter change, how far off your speedometer will read, Compare nominal dimensions and diameter change. This calculator cannot confirm fitment; verify your vehicle maker’s approved rim width, load and speed ratings, and clearance.
Tire size calculator
205/55R16
225/45R17
225/45R17 on a vehicle fitted with 205/55R16: Fitment not determined
Overall diameter changes 0.4%; section width changes 20 mm. Diameter comparison alone cannot confirm fitment. Verify the vehicle maker's approved rim width, tire load and speed ratings, and clearance at full steering lock and suspension compression. Confirm any possible ABS, traction-control, speedometer, and drivetrain effects with the vehicle maker or a qualified tire professional.
| 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
Overall diameter is one comparison point. It helps estimate speedometer change, but it cannot establish clearance, wheel compatibility, load capacity, or how vehicle systems will respond.
Green zone
A commonly used diameter guideline, not a fitment guarantee. A small diameter change does not confirm wheel compatibility, clearance, load capacity, or electronic-system behavior.
Yellow zone
A larger diameter change can affect indicated speed. Verify approved wheel and tire specifications, clearance through steering and suspension travel, and any vehicle-system implications.
Red zone
A large diameter change may affect speed indication and available clearance. Consult the vehicle maker or a qualified tire professional about compatibility and any required calibration.
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.
195/65R15 vs 205/55R16
Diameter −0.4% · Fitment not determined
205/55R16 vs 225/45R17
Diameter +0.4% · Fitment not determined
225/45R17 vs 225/40R18
Diameter +0.5% · Fitment not determined
225/45R17 vs 235/40R18
Diameter +1.7% · Fitment not determined
225/45R17 vs 245/40R18
Diameter +3.0% · Fitment not determined
235/45R18 vs 245/40R19
Diameter +1.5% · Fitment not determined
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?
- There is no universal diameter percentage that guarantees fitment or preserves vehicle-system behavior. A 3% change is sometimes used as a rule of thumb, but the vehicle maker’s approved wheel and tire specifications, load and speed ratings, and actual clearance determine suitability. Confirm possible effects on speedometer, ABS, stability control, and drivetrain with the vehicle maker or a qualified tire professional.
- Does changing tire size affect the speedometer?
- The calculator estimates speedometer change from nominal tire diameters. A larger nominal diameter generally means fewer revolutions per mile, so actual speed may be higher than indicated; a smaller diameter may have the opposite effect. Mounted and loaded dimensions vary, so verify against manufacturer data.
- How do I read a tire size like 225/45R17?
- 225 is the tread width in millimeters. 45 is the aspect ratio, meaning the sidewall is 45% of that width, so about 101 mm. R is radial construction. 17 is the wheel diameter in inches.
- Can I fit wider tires on the same wheels?
- Only when the tire and wheel manufacturers list the combination as compatible. Confirm the wheel’s approved width range, tire load and speed ratings, and clearance at full steering lock and suspension compression. A wider size at the same aspect ratio also changes nominal diameter.
How these numbers are calculated
Every figure on this site is worked out from the tire size code itself. No lookup table, 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 millimeters 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.