Tire Size Box

245/35R21 vs 245/40R21

245/40R21 compared with 245/35R21

+3.5% diameter, 0.0" width

245/40R21 is 28.72" tall vs 27.75". More than the 3% rule of thumb, so check carefully before switching.

27.8"21" rim3.4"9.6" wide
245/35R21 (solid) compared with 245/40R21 (dashed outline), drawn to scale

With 245/40R21 tires, when your speedometer says 60 mph you are really going

62.1 mph

245/40R21 is +0.96 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/35R21245/40R21Difference%
Overall diameter27.75"28.72"+0.96"+3.5%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.38"3.86"+0.48"+14.3%
Circumference87.19"90.22"+3.03"+3.5%
Revolutions per mile726.7702.3−24.4−3.4%

Speedometer difference

If your vehicle came with 245/35R21 and you fit 245/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 245/35R21 or 245/40R21?
245/40R21 is taller by 0.96 inches (28.72" vs 27.75"). 245/35R21 is the same width.
Can I replace 245/35R21 with 245/40R21?
It is not recommended without checking carefully. The diameter difference is +3.5%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 245/40R21 change my speedometer?
When your speedometer reads 60 mph with 245/40R21 fitted (calibrated for 245/35R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 245/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

Other comparisons