245/50R17 vs 225/50R18

Q: Can I replace 245/50R17 with 225/50R18?

225/50R18 is a safe swap for 245/50R17 by the usual ±3% rule: the diameter differs by +0.80%.

225/50R18 is a safe swap for 245/50R17 by the usual ±3% rule: the diameter differs by +0.80%.

245/50R17 26.65" · 225/50R18 26.86" · drawn to scale

Side by side

	245/50R17	225/50R18	Difference
Overall diameter	26.65"	26.86"	+0.21" (+0.80%)
Section width	245 mm	225 mm	-20 mm
Sidewall	4.82"	4.43"	-0.39"
Rim	17"	18"	+1"
Circumference	83.7"	84.4"	+0.7"
Revs per mile	757	751	-6
Ride height	—	—	+0.11"
Speedometer reads 60 mph	60.0	60.5	+0.5 mph

Speedometer and odometer

With 225/50R18 on a car calibrated for 245/50R17, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.4 mph
60 mph	60.5 mph
70 mph	70.6 mph
80 mph	80.6 mph

Other sizes close to 245/50R17

Questions

Can I replace 245/50R17 with 225/50R18?

225/50R18 is a safe swap for 245/50R17 by the usual ±3% rule: the diameter differs by +0.80%.

What's the difference between 245/50R17 and 225/50R18?

225/50R18 is 0.21" taller and 20 mm narrower; the sidewall is -0.39". At an indicated 60 mph you'd actually be doing 60.5 mph.

Will 225/50R18 fit on 245/50R17 wheels?

No — 245/50R17 needs a 17" wheel and 225/50R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.