235/45R18 vs 255/45R18

Q: Can I replace 235/45R18 with 255/45R18?

255/45R18 is a safe swap for 235/45R18 by the usual ±3% rule: the diameter differs by +2.69%.

255/45R18 is a safe swap for 235/45R18 by the usual ±3% rule: the diameter differs by +2.69%.

235/45R18 26.33" · 255/45R18 27.04" · drawn to scale

Side by side

	235/45R18	255/45R18	Difference
Overall diameter	26.33"	27.04"	+0.71" (+2.69%)
Section width	235 mm	255 mm	+20 mm
Sidewall	4.16"	4.52"	+0.35"
Rim	18"	18"	same
Circumference	82.7"	84.9"	+2.2"
Revs per mile	766	746	-20
Ride height	—	—	+0.35"
Speedometer reads 60 mph	60.0	61.6	+1.6 mph

Speedometer and odometer

With 255/45R18 on a car calibrated for 235/45R18, the speedometer reads 58.4 mph when you're really doing 60, and the odometer under-counts by 2.7%.

Speedo shows	Actual speed
30 mph	30.8 mph
45 mph	46.2 mph
60 mph	61.6 mph
70 mph	71.9 mph
80 mph	82.2 mph

Other sizes close to 235/45R18

Questions

Can I replace 235/45R18 with 255/45R18?

255/45R18 is a safe swap for 235/45R18 by the usual ±3% rule: the diameter differs by +2.69%.

What's the difference between 235/45R18 and 255/45R18?

255/45R18 is 0.71" taller and 20 mm wider; the sidewall is +0.35". At an indicated 60 mph you'd actually be doing 61.6 mph.

Will 255/45R18 fit on 235/45R18 wheels?

Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.