265/35R21 vs 245/40R21

Q: Can I replace 265/35R21 with 245/40R21?

245/40R21 is a safe swap for 265/35R21 by the usual ±3% rule: the diameter differs by +1.46%.

245/40R21 is a safe swap for 265/35R21 by the usual ±3% rule: the diameter differs by +1.46%.

265/35R21 28.30" · 245/40R21 28.72" · drawn to scale

Side by side

	265/35R21	245/40R21	Difference
Overall diameter	28.30"	28.72"	+0.41" (+1.46%)
Section width	265 mm	245 mm	-20 mm
Sidewall	3.65"	3.86"	+0.21"
Rim	21"	21"	same
Circumference	88.9"	90.2"	+1.3"
Revs per mile	713	702	-10
Ride height	—	—	+0.21"
Speedometer reads 60 mph	60.0	60.9	+0.9 mph

Speedometer and odometer

With 245/40R21 on a car calibrated for 265/35R21, 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 265/35R21

Questions

Can I replace 265/35R21 with 245/40R21?

245/40R21 is a safe swap for 265/35R21 by the usual ±3% rule: the diameter differs by +1.46%.

What's the difference between 265/35R21 and 245/40R21?

245/40R21 is 0.41" taller and 20 mm narrower; the sidewall is +0.21". At an indicated 60 mph you'd actually be doing 60.9 mph.

Will 245/40R21 fit on 265/35R21 wheels?

Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.