305/40R20 vs 255/50R20

Q: Can I replace 305/40R20 with 255/50R20?

255/50R20 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +1.46%.

255/50R20 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +1.46%.

305/40R20 29.61" · 255/50R20 30.04" · drawn to scale

Side by side

	305/40R20	255/50R20	Difference
Overall diameter	29.61"	30.04"	+0.43" (+1.46%)
Section width	305 mm	255 mm	-50 mm
Sidewall	4.80"	5.02"	+0.22"
Rim	20"	20"	same
Circumference	93.0"	94.4"	+1.4"
Revs per mile	681	671	-10
Ride height	—	—	+0.22"
Speedometer reads 60 mph	60.0	60.9	+0.9 mph

Speedometer and odometer

With 255/50R20 on a car calibrated for 305/40R20, the speedometer reads 59.1 mph when you're really doing 60, and the odometer under-counts by 1.5%.

Speedo shows	Actual speed
30 mph	30.4 mph
45 mph	45.7 mph
60 mph	60.9 mph
70 mph	71.0 mph
80 mph	81.2 mph

Other sizes close to 305/40R20

Questions

Can I replace 305/40R20 with 255/50R20?

255/50R20 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +1.46%.

What's the difference between 305/40R20 and 255/50R20?

255/50R20 is 0.43" taller and 50 mm narrower; the sidewall is +0.22". At an indicated 60 mph you'd actually be doing 60.9 mph.

Will 255/50R20 fit on 305/40R20 wheels?

Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.