245/40R21 vs 285/35R21

Q: Can I replace 245/40R21 with 285/35R21?

285/35R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.48%.

285/35R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.48%.

245/40R21 28.72" · 285/35R21 28.85" · drawn to scale

Side by side

	245/40R21	285/35R21	Difference
Overall diameter	28.72"	28.85"	+0.14" (+0.48%)
Section width	245 mm	285 mm	+40 mm
Sidewall	3.86"	3.93"	+0.07"
Rim	21"	21"	same
Circumference	90.2"	90.6"	+0.4"
Revs per mile	702	699	-3
Ride height	—	—	+0.07"
Speedometer reads 60 mph	60.0	60.3	+0.3 mph

Speedometer and odometer

With 285/35R21 on a car calibrated for 245/40R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.5%.

Speedo shows	Actual speed
30 mph	30.1 mph
45 mph	45.2 mph
60 mph	60.3 mph
70 mph	70.3 mph
80 mph	80.4 mph

Other sizes close to 245/40R21

Questions

Can I replace 245/40R21 with 285/35R21?

285/35R21 is a safe swap for 245/40R21 by the usual ±3% rule: the diameter differs by +0.48%.

What's the difference between 245/40R21 and 285/35R21?

285/35R21 is 0.14" taller and 40 mm wider; the sidewall is +0.07". At an indicated 60 mph you'd actually be doing 60.3 mph.

Will 285/35R21 fit on 245/40R21 wheels?

Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.