245/40R21 vs 255/40R21

Q: Can I replace 245/40R21 with 255/40R21?

255/40R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +1.10%.

255/40R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +1.10%.

245/40R21 28.72" · 255/40R21 29.03" · drawn to scale

Side by side

	245/40R21	255/40R21	Difference
Overall diameter	28.72"	29.03"	+0.31" (+1.10%)
Section width	245 mm	255 mm	+10 mm
Sidewall	3.86"	4.02"	+0.16"
Rim	21"	21"	same
Circumference	90.2"	91.2"	+1.0"
Revs per mile	702	695	-8
Ride height	—	—	+0.16"
Speedometer reads 60 mph	60.0	60.7	+0.7 mph

Speedometer and odometer

With 255/40R21 on a car calibrated for 245/40R21, the speedometer reads 59.3 mph when you're really doing 60, and the odometer under-counts by 1.1%.

Speedo shows	Actual speed
30 mph	30.3 mph
45 mph	45.5 mph
60 mph	60.7 mph
70 mph	70.8 mph
80 mph	80.9 mph

Other sizes close to 245/40R21

Questions

Can I replace 245/40R21 with 255/40R21?

255/40R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +1.10%.

What's the difference between 245/40R21 and 255/40R21?

255/40R21 is 0.31" taller and 10 mm wider; the sidewall is +0.16". At an indicated 60 mph you'd actually be doing 60.7 mph.

Will 255/40R21 fit on 245/40R21 wheels?

Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.