Tire Size Comparison Calculator
Diameter, difference percent, and speedometer error between two tire sizes.
What it does
Tire sidewalls print everything needed: width in millimeters, aspect ratio as sidewall percent of width, and rim diameter in inches. Diameter sums the rim plus two sidewalls, and comparing two sizes shows the percent change that gearing, clearance, and speedometers feel.
The tool above takes current and new sizes. It returns both diameters, the percent difference, and the speedometer reading at a true 60 mph.
Three percent is the trade guide. Inside it, gearing, ABS, and arches cope fine. Past it, speedometers lie, gearing stretches, and tires may rub at full lock or load.
Wider is not automatically grippier in all weather. Contact patch shape changes with width, and narrow tires often bite better in rain and snow.
How to use
- Read the three numbers off each sidewall.
- Enter current size first, new size second.
- Check the percent stays within 3%.
- Confirm load rating and rim width fit the new tire.
Formula
- rim x 25.4
- Rim inches converted to millimeters.
- width x aspect
- One sidewall height in mm. Doubled top and bottom.
- new / old
- Rolling ratio scaling speedometer and gearing.
Circumference scales with diameter, so revolutions per mile equal 63,360 divided by diameter in inches. Gearing and odometers follow the same ratio as the speedometer.
Worked example
Example: 205/55R16 to 225/50R16
Current 205/55R16, new 225/50R16.
- Old: 16 x 25.4 + 2 x 205 x 0.55 = 406.4 + 225.5 = 631.9 mm.
- New: 16 x 25.4 + 2 x 225 x 0.50 = 406.4 + 225.0 = 631.4 mm.
- Difference: (631.4 - 631.9) / 631.9 = -0.08%.
- Speedo: 60 x 631.4 / 631.9 = 59.95 mph at a true 60. Safe swap.
Reference table
| New size | Diameter | Difference |
|---|---|---|
| 195/60R16 | 640.4 mm | +1.35% |
| 215/55R16 | 642.9 mm | +1.74% |
| 225/50R16 | 631.4 mm | -0.08% |
| 225/45R17 | 634.3 mm | +0.38% |
Limits
- Diameter math only. Load, speed rating, and rim width need separate checks.
- Worn versus new tread shifts diameter near 2%.
- Pressure changes rolling radius slightly.
- Suspension and arches decide real clearance.
Common mistakes
- Reading aspect ratio as millimeters instead of percent.
- Forgetting the sidewall counts twice, top and bottom.
- Mixing rim diameters without checking brake clearance.
- Ignoring load index on heavier vehicles.
- Skipping alignment after size changes.
Try it yourself
Quick checks. Answers sit tucked under each question.
Diameter of 195/65R15?
15 x 25.4 + 2 x 195 x 0.65 = 381 + 253.5 = 634.5 mm.
Speedo with +2% larger tires at true 60?
60 / 1.02 = 58.82 mph shown.
Questions
What does 205/55R16 mean?
205 mm wide, sidewall 55% of width, radial, 16 inch rim.
How much bigger can I go?
Stay within 3% diameter. Past that, gearing and clearance suffer.
Do bigger tires slow the speedometer?
Yes. Larger rolling diameter covers more ground per turn, so readings drop.
Can I change rim size?
Yes with matching diameter: higher rim plus lower profile, called plus sizing.
Does width affect the speedometer?
Only through diameter. Same diameter means same readings at any width.
Winter tires narrower?
Often one step narrower. They cut through slush better with the same diameter.