235/40R17 vs 205/40R18

Q: Can I replace 235/40R17 with 205/40R18?

205/40R18 is a safe swap for 235/40R17 by the usual ±3% rule: the diameter differs by +0.23%.

205/40R18 is a safe swap for 235/40R17 by the usual ±3% rule: the diameter differs by +0.23%.

235/40R17 24.40" · 205/40R18 24.46" · drawn to scale

Side by side

	235/40R17	205/40R18	Difference
Overall diameter	24.40"	24.46"	+0.06" (+0.23%)
Section width	235 mm	205 mm	-30 mm
Sidewall	3.70"	3.23"	-0.47"
Rim	17"	18"	+1"
Circumference	76.7"	76.8"	+0.2"
Revs per mile	827	825	-2
Ride height	—	—	+0.03"
Speedometer reads 60 mph	60.0	60.1	+0.1 mph

Speedometer and odometer

With 205/40R18 on a car calibrated for 235/40R17, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.2%.

Speedo shows	Actual speed
30 mph	30.1 mph
45 mph	45.1 mph
60 mph	60.1 mph
70 mph	70.2 mph
80 mph	80.2 mph

Other sizes close to 235/40R17

Questions

Can I replace 235/40R17 with 205/40R18?

205/40R18 is a safe swap for 235/40R17 by the usual ±3% rule: the diameter differs by +0.23%.

What's the difference between 235/40R17 and 205/40R18?

205/40R18 is 0.06" taller and 30 mm narrower; the sidewall is -0.47". At an indicated 60 mph you'd actually be doing 60.1 mph.

Will 205/40R18 fit on 235/40R17 wheels?

No — 235/40R17 needs a 17" wheel and 205/40R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.