Tire Size Box

235/35R20 vs 235/40R20

235/40R20 compared with 235/35R20

+3.5% diameter, 0.0" width

235/40R20 is 27.40" tall vs 26.48". More than the 3% rule of thumb, so check carefully before switching.

26.5"20" rim3.2"9.3" wide
235/35R20 (solid) compared with 235/40R20 (dashed outline), drawn to scale

With 235/40R20 tires, when your speedometer says 60 mph you are really going

62.1 mph

235/40R20 is +0.93 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/35R20235/40R20Difference%
Overall diameter26.48"27.40"+0.93"+3.5%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.24"3.70"+0.46"+14.3%
Circumference83.18"86.08"+2.91"+3.5%
Revolutions per mile761.7736.0−25.7−3.4%

Speedometer difference

If your vehicle came with 235/35R20 and you fit 235/40R20, 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, 235/35R20 or 235/40R20?
235/40R20 is taller by 0.93 inches (27.40" vs 26.48"). 235/35R20 is the same width.
Can I replace 235/35R20 with 235/40R20?
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 235/40R20 change my speedometer?
When your speedometer reads 60 mph with 235/40R20 fitted (calibrated for 235/35R20), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 235/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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