Tire Size Box

245/35R20 vs 245/40R20

245/40R20 compared with 245/35R20

+3.6% diameter, 0.0" width

245/40R20 is 27.72" tall vs 26.75". More than the 3% rule of thumb, so check carefully before switching.

26.8"20" rim3.4"9.6" wide
245/35R20 (solid) compared with 245/40R20 (dashed outline), drawn to scale

With 245/40R20 tires, when your speedometer says 60 mph you are really going

62.2 mph

245/40R20 is +0.96 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/35R20245/40R20Difference%
Overall diameter26.75"27.72"+0.96"+3.6%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.38"3.86"+0.48"+14.3%
Circumference84.04"87.07"+3.03"+3.6%
Revolutions per mile753.9727.7−26.2−3.5%

Speedometer difference

If your vehicle came with 245/35R20 and you fit 245/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 245/35R20 or 245/40R20?
245/40R20 is taller by 0.96 inches (27.72" vs 26.75"). 245/35R20 is the same width.
Can I replace 245/35R20 with 245/40R20?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 245/40R20 change my speedometer?
When your speedometer reads 60 mph with 245/40R20 fitted (calibrated for 245/35R20), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 245/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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