245/35R21 vs 255/40R20

Q: Can I replace 245/35R21 with 255/40R20?

255/40R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +1.01%.

255/40R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +1.01%.

245/35R21 27.75" · 255/40R20 28.03" · drawn to scale

Side by side

	245/35R21	255/40R20	Difference
Overall diameter	27.75"	28.03"	+0.28" (+1.01%)
Section width	245 mm	255 mm	+10 mm
Sidewall	3.38"	4.02"	+0.64"
Rim	21"	20"	-1"
Circumference	87.2"	88.1"	+0.9"
Revs per mile	727	719	-7
Ride height	—	—	+0.14"
Speedometer reads 60 mph	60.0	60.6	+0.6 mph

Speedometer and odometer

With 255/40R20 on a car calibrated for 245/35R21, the speedometer reads 59.4 mph when you're really doing 60, and the odometer under-counts by 1.0%.

Speedo shows	Actual speed
30 mph	30.3 mph
45 mph	45.5 mph
60 mph	60.6 mph
70 mph	70.7 mph
80 mph	80.8 mph

Other sizes close to 245/35R21

Questions

Can I replace 245/35R21 with 255/40R20?

255/40R20 is a safe swap for 245/35R21 by the usual ±3% rule: the diameter differs by +1.01%.

What's the difference between 245/35R21 and 255/40R20?

255/40R20 is 0.28" taller and 10 mm wider; the sidewall is +0.64". At an indicated 60 mph you'd actually be doing 60.6 mph.

Will 255/40R20 fit on 245/35R21 wheels?

No — 245/35R21 needs a 21" wheel and 255/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.