305/35R18 vs 305/40R18
Comparison of overall diameter, sidewall, speedometer error, and ride-height change between these two tire sizes.
305/35R18
- Section width
- 305 mm
- Aspect ratio
- 35%
- Rim
- 18"
- Sidewall
- 106.8 mm
- Overall diameter
- 670.7 mm
- Revs / mile
- 764
305/40R18
- Section width
- 305 mm
- Aspect ratio
- 40%
- Rim
- 18"
- Sidewall
- 122.0 mm
- Overall diameter
- 701.2 mm
- Revs / mile
- 731
Difference
- Overall diameter
- +30.5 mm (+4.55%)
- Sidewall
- +15.3 mm
- Tread width
- +0 mm
- Ride height (axle)
- +15.3 mm
- Speedometer error
- +4.55%
- At indicated 60 mph
- actual 62.73 mph
Will 305/40R18 fit instead of 305/35R18?
The overall diameter changes by 4.55%. Most manufacturers consider a swap acceptable when the difference stays under 3% — beyond that, you may see speedometer error, ABS/TPMS warnings, or wheel-well clearance problems. This page only covers the math; your vehicle's wheel arches, suspension geometry, and load rating still matter.
What the numbers mean
- Overall diameter is the outside-to-outside height of the tire. Affects ride height, gearing, and speedometer.
- Sidewall is the height of the rubber from rim to tread. Lower sidewall = sharper steering, harsher ride.
- Speedometer error is the percentage your indicated speed will be off after the swap (positive = speedo reads lower than actual).