245/45R18 vs 245/40R19
Side-by-side comparison of overall diameter, sidewall height, speedometer error, and ride-height change.
Safe to swap — Diameter difference is under 1.5% — within normal tolerance.
To scale
245/45R18
- Section width
- 245 mm
- Aspect ratio
- 45%
- Rim diameter
- 18"
- Sidewall
- 110.3 mm
- Overall diameter
- 677.7 mm
- Circumference
- 2129 mm
- Revs / mile
- 756
- Revs / km
- 470
245/40R19
- Section width
- 245 mm
- Aspect ratio
- 40%
- Rim diameter
- 19"
- Sidewall
- 98.0 mm
- Overall diameter
- 678.6 mm
- Circumference
- 2132 mm
- Revs / mile
- 755
- Revs / km
- 469
Difference
- Overall diameter
- +0.9 mm (+0.13%)
- Sidewall height
- -12.3 mm
- Tread width
- +0 mm
- Ride height (axle)
- +0.4 mm
- Speedometer error
- +0.13%
- At indicated 60 mph
- actual 60.1 mph
- At indicated 100 km/h
- actual 100.1 km/h
Will 245/40R19 fit instead of 245/45R18?
The overall diameter changes by 0.13%. 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 — outside-to-outside height of the tire. Affects ride height, gearing, and speedometer calibration.
- Sidewall height — rubber from rim edge to tread. Lower sidewall gives sharper steering response but a harsher ride.
- Speedometer error — how far off your indicated speed will be after the swap. Positive means the speedo under-reads (actual speed is higher than shown).