A tire size calculator compares two tire sizes by their overall diameter and shows how much bigger or smaller the new tire is. Formula: Diameter mm = rim in x 25.4 + 2 x (width mm x aspect/100). Difference % = (d2 – d1) / d1 x 100. Going from 225/45R17 to 245/40R18 makes the tire about 2.98% taller.
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How to use the tire size calculator
- Enter the width, aspect ratio and rim size of the current tire.
- Enter the same three numbers for the new tire.
- Read both diameters and the percentage difference.
How the tire size calculator works
The tire size calculator reads the three numbers in a metric tire size, such as 225/45R17. The first is the section width in millimeters, the second is the aspect ratio (sidewall height as a percentage of width), and the third is the rim diameter in inches. Overall diameter in mm = rim × 25.4 + 2 × width × aspect ÷ 100. The tool compares two sizes and shows the percentage difference in diameter.
Tire size calculator worked example
A 225/45R17 tire has a sidewall of 225 × 0.45 = 101.25 mm and a rim of 431.8 mm, so its diameter is 634.3 mm. A 245/40R18 is 653.2 mm. The tire size calculator shows +2.98%, meaning the new tire is about 3% taller.
| Size 1 | Size 2 | Dia. 1 (mm) | Dia. 2 (mm) | Difference |
|---|---|---|---|---|
| 225/45R17 | 245/40R18 | 634.3 | 653.2 | +2.98% |
| 205/55R16 | 215/55R16 | 631.9 | 642.9 | +1.74% |
| 265/70R17 | 285/70R17 | 802.8 | 830.8 | +3.49% |
| 225/45R17 | 225/40R18 | 634.3 | 637.2 | +0.46% |
| 195/65R15 | 205/55R16 | 634.5 | 631.9 | −0.41% |
Speedometer effect
A taller tire travels farther each turn, so the speedometer reads low. With a 2.98% larger diameter, when the speedometer shows 60 mph you are really doing about 61.8 mph. A smaller tire makes it read high. Many tire shops aim to keep the change within about 3% so that speedometer, odometer, ABS and stability systems behave as designed.
Clearance and fitment
Diameter is only part of the story. A wider tire may rub on suspension parts or fender liners, especially at full lock or under load. Wheel width and offset must suit the tire width. Load index and speed rating must be at least what the vehicle maker specifies. The tire size calculator does not check any of these.
Safety guidance
NHTSA advises buying tires that are the same size as the original tires or another size the vehicle maker recommends. The correct sizes and inflation pressures are listed on the Tire and Loading Information label on the driver’s door edge or in the owner’s manual. If in doubt, ask a tire dealer to confirm the fit before buying.
Inputs and limits
Enter width in mm, aspect as a percentage and rim in inches, for both tires. All six values must be greater than zero, and the aspect ratio cannot exceed 100; the tool rejects anything else. Flotation sizes such as 33×12.50R15 already state the diameter in inches; convert them to mm by multiplying by 25.4 if you want to compare them with a metric size.
Reading the full sidewall code
After the size, the sidewall shows a load index and a speed symbol, such as 91V. The load index maps to the maximum load per tire, and the speed symbol to the maximum sustained speed. P before the size marks a passenger tire and LT a light-truck tire, which use different load tables. The DOT code ends in four digits giving the week and year of manufacture, such as 0308 for the third week of 2008.
When you use the tire size calculator to compare a plus-size option, such as moving from a 17-inch to an 18-inch wheel, the aspect ratio usually drops so that overall diameter stays close. The table shows that 225/45R17 and 225/40R18 differ by less than half a percent, which is why that pairing is a common upgrade.
Tips
- Compare against the original size, not your current replacement.
- Keep diameter changes small, ideally within 3%.
- Check load index and speed rating on the sidewall.
For study and planning only. Check lab, engineering and safety work against your course, instrument or a qualified professional.
Related converters and calculators
Related calculators:
- Speed calculator: speed
- Gas mileage calculator: fuel economy
- mm to inches: units
- Auto loan calculator: car costs
Sources
NHTSA: Tire safety (TireWise); NIST: SI units – length.
Formula and method
Diameter mm = rim_in x 25.4 + 2 x (width_mm x aspect/100). Difference % = (d2 - d1) / d1 x 100.
Builds overall diameter for each size code, then reports how much larger or smaller size 2 is versus size 1.
Questions about this calculator
How is tire diameter calculated?
Rim × 25.4 + 2 × width × aspect ÷ 100, in mm.
What does 225/45R17 mean?
225 mm wide, sidewall 45% of width, 17-inch rim.
How much difference is safe?
Many shops aim for within about 3%.
How does a bigger tire affect the speedometer?
It reads lower than your true speed.
Does it check fitment?
No; check clearance, load index and speed rating.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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