245/40R18 vs 225/50R17

Q: Can I replace 245/40R18 with 225/50R17?

225/50R17 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +0.55%.

225/50R17 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +0.55%.

245/40R18 25.72" · 225/50R17 25.86" · drawn to scale

Side by side

	245/40R18	225/50R17	Difference
Overall diameter	25.72"	25.86"	+0.14" (+0.55%)
Section width	245 mm	225 mm	-20 mm
Sidewall	3.86"	4.43"	+0.57"
Rim	18"	17"	-1"
Circumference	80.8"	81.2"	+0.4"
Revs per mile	784	780	-4
Ride height	—	—	+0.07"
Speedometer reads 60 mph	60.0	60.3	+0.3 mph

Speedometer and odometer

With 225/50R17 on a car calibrated for 245/40R18, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.6%.

Speedo shows	Actual speed
30 mph	30.2 mph
45 mph	45.2 mph
60 mph	60.3 mph
70 mph	70.4 mph
80 mph	80.4 mph

Other sizes close to 245/40R18

Questions

Can I replace 245/40R18 with 225/50R17?

225/50R17 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +0.55%.

What's the difference between 245/40R18 and 225/50R17?

225/50R17 is 0.14" taller and 20 mm narrower; the sidewall is +0.57". At an indicated 60 mph you'd actually be doing 60.3 mph.

Will 225/50R17 fit on 245/40R18 wheels?

No — 245/40R18 needs a 18" wheel and 225/50R17 a 17" wheel. This is a plus/minus-size swap that changes wheels too.