255/40R19 vs 245/40R18
245/40R18 sits −4.9% away from 255/40R19 on rolling diameter — past the 3% rule but short of 5%. It will physically fit many cars, but you are trading measurable speedometer and odometer error for the look. Budget 16.7 mm of extra clearance per side, and check the inner arch at full lock before committing. On a car with torque-vectoring or adaptive cruise, have the wheel-speed calibration checked.
- Diameter change
- −4.86%
- −33.4 mm
- Ride height
- −16.7 mm
- half the diameter change
- Tread width
- −10 mm
- 245 mm section
- Speedo at 60
- 57.1
- true mph when the dial reads 60
Side by side
| Measure | 255/40R19 | 245/40R18 | Change |
|---|---|---|---|
| Overall diameter | 27.03″ | 25.72″ | −4.86% |
| Section width | 255 mm | 245 mm | −10 mm |
| Sidewall height | 102.0 mm | 98.0 mm | −4.0 mm |
| Rim diameter | 19″ | 18″ | −1″ |
| Revolutions / mile | 746 | 784 | +38 |
Questions about this swap
›Can I fit 245/40R18 instead of 255/40R19?
Only with care. The diameter differs by −4.9%, outside the 3% rule, so clearance and speedometer accuracy both need checking first.
›How much will my speedometer be out with 245/40R18 instead of 255/40R19?
By about 4.9%. At an indicated 60 mph you would actually be doing 57.1 mph, so the dial over-reads your true speed.
›Does fitting 245/40R18 change ride height from 255/40R19?
Yes, by roughly 16.7 mm. Rolling radius is half the diameter change, so the axle sits that much lower and you need the same amount of extra room above the tyre.
›Will my odometer read differently on 245/40R18 rather than 255/40R19?
It will read 4.9% high. Over 10,000 recorded miles that is about 486 miles of difference against the true distance travelled.
Want to try other sizes? Open the calculator.