245/40R18 vs 215/50R18

Q: Can I replace 245/40R18 with 215/50R18?

215/50R18 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +2.91%.

215/50R18 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +2.91%.

245/40R18 25.72" · 215/50R18 26.46" · drawn to scale

Side by side

	245/40R18	215/50R18	Difference
Overall diameter	25.72"	26.46"	+0.75" (+2.91%)
Section width	245 mm	215 mm	-30 mm
Sidewall	3.86"	4.23"	+0.37"
Rim	18"	18"	same
Circumference	80.8"	83.1"	+2.4"
Revs per mile	784	762	-22
Ride height	—	—	+0.37"
Speedometer reads 60 mph	60.0	61.7	+1.7 mph

Speedometer and odometer

With 215/50R18 on a car calibrated for 245/40R18, the speedometer reads 58.3 mph when you're really doing 60, and the odometer under-counts by 2.9%.

Speedo shows	Actual speed
30 mph	30.9 mph
45 mph	46.3 mph
60 mph	61.7 mph
70 mph	72.0 mph
80 mph	82.3 mph

Other sizes close to 245/40R18

Questions

Can I replace 245/40R18 with 215/50R18?

215/50R18 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +2.91%.

What's the difference between 245/40R18 and 215/50R18?

215/50R18 is 0.75" taller and 30 mm narrower; the sidewall is +0.37". At an indicated 60 mph you'd actually be doing 61.7 mph.

Will 215/50R18 fit on 245/40R18 wheels?

Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 215 mm tire and the fender clearance.