Calculate tire dimensions, compare tire sizes, and check speedometer accuracy. Find equivalent tire sizes for your vehicle.
Size Difference
+0.43"
+1.7%
Click a preset to set as Original or New tire size:
185/65R15
225/45R17
235/55R18
275/55R20
265/70R17
245/35R19
225/45R17
Overall: 25.0" diameter, 78.5" circumference
235/40R18
Overall: 25.4" diameter, 79.8" circumference
Diameter Diff
+0.43"
Percentage
+1.7%
Speedo @30
Actual: 30.5 mph
Speedo @60
Actual: 61.0 mph
✓ These tires are within the recommended 3% size difference
| Specification | Original | New | Difference |
|---|---|---|---|
| Overall Diameter | 24.97" | 25.40" | +0.43" |
| Sidewall Height | 3.99" | 3.70" | -0.29" |
| Circumference | 78.45" | 79.80" | +1.35" |
| Revs/Mile | 808 | 794 | -14 |
| Width | 225 mm (8.9") | 235 mm (9.3") | +10 mm |
Original
225/45R17
New
235/40R18
When upsizing wheels, decrease aspect ratio to maintain similar overall diameter:
| Starting Size | Plus 1 | Plus 2 |
|---|---|---|
| 205/55R16 | 215/45R17 | 225/40R18 |
| 225/45R17 | 235/40R18 | 245/35R19 |
| 245/45R18 | 255/40R19 | 265/35R20 |
Tire size like 225/45R17: 225 = width in mm, 45 = aspect ratio (sidewall height as % of width), R = radial, 17 = wheel diameter in inches. So 225/45R17 means 225mm wide, sidewall is 45% of 225mm (101mm), fits 17" wheel.
Most manufacturers recommend staying within 3% of original overall diameter. Going outside this can affect speedometer accuracy, ABS/traction control, gear ratios, and may cause rubbing. Always verify clearance before fitting larger tires.
Yes. Larger tires rotate fewer times per mile, making the speedometer read LOW (you're actually going faster). Smaller tires read HIGH. A 3% larger tire at 60 mph indicated means you're actually doing ~62 mph.
Plus sizing increases wheel diameter while decreasing sidewall height to maintain similar overall diameter. Plus 1 = 1" larger wheels. Benefits: better handling, look. Drawbacks: harsher ride, more expensive, more prone to pothole damage.
Yes. Larger/heavier tires can reduce MPG by 1-3% due to increased rolling resistance and weight. Wider tires have more rolling resistance. Underinflated tires significantly hurt efficiency (1 PSI low = 0.2% worse MPG).
Size Difference
+0.43"
+1.7%