Tire Size Calculator
Current tire (stock)
225/45R17
New tire (compare)
235/40R18
| Spec | 225/45R17 | 235/40R18 | Difference |
|---|---|---|---|
| Section width | 8.86 in (225 mm) | 9.25 in (235 mm) | +10 mm |
| Sidewall height | 3.99 in (101.3 mm) | 3.70 in (94.0 mm) | -0.29 in |
| Overall diameter | 24.97 in (634.3 mm) | 25.40 in (645.2 mm) | +1.72 % |
| Circumference | 78.45 in | 79.80 in | 1.35 in |
| Revolutions per mile | 808 | 794 | -14 |
Diameter difference
+1.72 %
Within the usual ±3 % guideline.
Actual speed on the new tires
61.0 mph
How the tire dimensions are calculated
Worked example: 225/45R17. Sidewall = 225 × 0.45 = 101.3 mm = 3.99 in. Diameter = 17 + 2 × 3.99 = 24.97 in. Circumference = 78.4 in, so the tire turns about 808 times per mile. Swapping to 235/40R18: sidewall 3.70 in, diameter 25.40 in — +1.7 %, well inside the ±3 % rule, and the speedometer will read 60 mph when you are doing 61.0.
These are nominal figures from the size markings. Real tires vary by brand and model by a few tenths of an inch, and the loaded rolling radius is a little smaller than the unloaded diameter, so manufacturers’ published revolutions-per-mile numbers are typically 1–3 % higher than the nominal calculation.
Common OEM sizes and their diameters
| Size | Diameter | Typical vehicle |
|---|---|---|
| 205/55R16 | 24.9 in | Civic, Corolla, Golf |
| 225/45R17 | 25.0 in | Accord, Camry, 3 Series |
| 235/40R18 | 25.4 in | Plus-one fitment of the above |
| 235/65R17 | 29.0 in | CR-V, RAV4, Escape |
| 265/70R17 | 31.6 in | F-150, Silverado, Tacoma (“32s”) |
| 275/55R20 | 31.9 in | Full-size trucks, 20-in wheels |
| 285/70R17 | 32.7 in | Common off-road upgrade (“33s”) |
| 35×12.50R17 | 35.0 in | Flotation size: diameter × width in inches |
Before you change size
Keep the overall diameter within ±3 % unless you plan to recalibrate the speedometer. Check the load index and speed rating on the new size against the door-jamb placard, make sure the wheel width is within the tire’s approved rim range, and confirm clearance at full steering lock and full suspension compression. Wider tires with a lower profile grip better on dry pavement but ride harder and are more prone to rim damage from potholes.
Frequently asked questions
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