275/35R20 vs 255/40R20

Q: Can I replace 275/35R20 with 255/40R20?

255/40R20 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +1.64%.

255/40R20 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +1.64%.

275/35R20 27.58" · 255/40R20 28.03" · drawn to scale

Side by side

	275/35R20	255/40R20	Difference
Overall diameter	27.58"	28.03"	+0.45" (+1.64%)
Section width	275 mm	255 mm	-20 mm
Sidewall	3.79"	4.02"	+0.23"
Rim	20"	20"	same
Circumference	86.6"	88.1"	+1.4"
Revs per mile	731	719	-12
Ride height	—	—	+0.23"
Speedometer reads 60 mph	60.0	61.0	+1.0 mph

Speedometer and odometer

With 255/40R20 on a car calibrated for 275/35R20, the speedometer reads 59.0 mph when you're really doing 60, and the odometer under-counts by 1.6%.

Speedo shows	Actual speed
30 mph	30.5 mph
45 mph	45.7 mph
60 mph	61.0 mph
70 mph	71.1 mph
80 mph	81.3 mph

Other sizes close to 275/35R20

Questions

Can I replace 275/35R20 with 255/40R20?

255/40R20 is a safe swap for 275/35R20 by the usual ±3% rule: the diameter differs by +1.64%.

What's the difference between 275/35R20 and 255/40R20?

255/40R20 is 0.45" taller and 20 mm narrower; the sidewall is +0.23". At an indicated 60 mph you'd actually be doing 61.0 mph.

Will 255/40R20 fit on 275/35R20 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.