Tire Size Calculator
Compare two tire sizes for diameter, speedometer error and clearance before you commit to a change.
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Current tires
New tires
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This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Diameter difference
+1.06"
+3.71% — outside the usual 3% tolerance
Includes your inputs, the full breakdown, and every row of the table.
Current diameter
28.52"
New diameter
29.57"
Real speed at 70 mph
72.6 mph
Odometer error
+3.71%
- Current tire
- 225/65R17
- New tire
- 245/60R18
- Diameter change
- +1.06" (3.71%)
- Clearance needed at the top
- +0.53"
- Width change
- +20 mm
- Sidewall change
- 0.03"
- Speedometer reads 70, you are doing
- 72.6 mph
| Measure | Current | New | Change |
|---|---|---|---|
| Size | 225/65R17 | 245/60R18 | — |
| Overall diameter | 28.52" | 29.57" | +1.06" |
| Sidewall height | 5.76" | 5.79" | 0.03" |
| Section width | 225 mm | 245 mm | +20 mm |
| Circumference | 89.58" | 92.91" | 3.33" |
| Revolutions per mile | 707 | 682 | -25 |
What this means
- A 3.71% diameter change exceeds the three percent tolerance normally accepted. Expect noticeable speedometer and odometer error, and check clearance carefully at full lock and full suspension compression.
- Larger tires make the speedometer read low — at an indicated 70 mph you would actually be doing 72.6 mph, which matters for speed limits and for warranty mileage.
- A 20 mm width change also needs the right wheel width and enough clearance at full steering lock. Check the tire's approved rim width range before ordering.
How the tire size calculation works
A tire size like 225/65R17 encodes three things: section width in millimetres, aspect ratio as a percentage of that width, and wheel diameter in inches. It is a genuinely awkward mixed-unit convention, and the aspect ratio being relative rather than absolute is what catches people out — changing the width alone changes the sidewall height too, because sixty-five percent of a wider tire is a taller sidewall.
Overall diameter is what matters most when changing sizes, because your speedometer, odometer, ABS and traction control all infer road speed from wheel rotation. A larger tire covers more ground per revolution, so the car travels faster than the speedometer indicates and accumulates fewer recorded miles than it actually drives. Three percent is the tolerance most manufacturers and fitters work to; beyond that the error becomes significant and stability systems can behave oddly.
Plus sizing — fitting a larger wheel with a proportionally shorter sidewall to keep overall diameter constant — is the standard way to change the look and handling without breaking anything. Larger wheels with less sidewall give sharper steering response and more precise turn-in, at the cost of ride comfort, greater vulnerability to pothole damage, and more unsprung weight. That trade-off is real and gets worse the further you take it.
Frequently asked questions
How much bigger can I go without problems?
Within three percent of the original diameter is the widely used tolerance, and it keeps speedometer error small and stability systems happy. Beyond that you should also check physical clearance at full steering lock and full suspension compression, which is where rubbing usually shows up first.
Will bigger tires mess up my speedometer?
Yes, proportionally to the diameter change. Larger tires make the speedometer read low, so you are travelling faster than indicated, and your odometer under-records mileage. Some vehicles can be recalibrated; many cannot without aftermarket hardware.
What does 225/65R17 actually mean?
The tire is 225 mm wide at its widest point, the sidewall height is 65% of that width, and it fits a 17-inch wheel. The R indicates radial construction. Overall diameter works out at 17 plus twice the sidewall, converted to inches.
Is plus sizing worth it?
It depends what you want. A larger wheel with a shorter sidewall sharpens steering response and looks better to most eyes, but rides harder, damages more easily on potholes, and usually costs more to replace. Keeping overall diameter constant is what makes it safe.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.