Click a preset to set as Original or New tire size:
185/65R15
225/45R17
235/55R18
275/55R20
265/70R17
245/35R19
Original Tire Size
225/45R17
Overall: 25.0" diameter, 78.5" circumference
New Tire Size
235/40R18
Overall: 25.4" diameter, 79.8" circumference
Comparison Result
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
Detailed Specifications
| 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 |
Visual Comparison
Original
225/45R17
New
235/40R18
Plus Sizing Guide
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 |
Frequently Asked Questions
How do I read tire size?
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.
How much can tire size differ from original?
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.
Will bigger tires affect my speedometer?
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.
What is plus sizing?
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.
Does tire size affect fuel economy?
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).