Tire Size Box

235/30R20 vs 235/40R20

235/40R20 compared with 235/30R20

+7.2% diameter, 0.0" width

235/40R20 is 27.40" tall vs 25.55". More than the 3% rule of thumb, so check carefully before switching.

25.6"20" rim2.8"9.3" wide
235/30R20 (solid) compared with 235/40R20 (dashed outline), drawn to scale

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

64.3 mph

235/40R20 is +1.85 inches (+7.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/30R20235/40R20Difference%
Overall diameter25.55"27.40"+1.85"+7.2%
Section width9.25"9.25"0.00"0.0%
Sidewall height2.78"3.70"+0.93"+33.3%
Circumference80.27"86.08"+5.81"+7.2%
Revolutions per mile789.3736.0−53.3−6.8%

Speedometer difference

If your vehicle came with 235/30R20 and you fit 235/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.2 mph+2.2 mph
40 mph42.9 mph+2.9 mph
50 mph53.6 mph+3.6 mph
60 mph64.3 mph+4.3 mph
65 mph69.7 mph+4.7 mph
70 mph75.1 mph+5.1 mph
75 mph80.4 mph+5.4 mph
80 mph85.8 mph+5.8 mph

Each size on its own

Questions

Which is bigger, 235/30R20 or 235/40R20?
235/40R20 is taller by 1.85 inches (27.40" vs 25.55"). 235/30R20 is the same width.
Can I replace 235/30R20 with 235/40R20?
It is not recommended without checking carefully. The diameter difference is +7.2%, 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 235/30R20), your real speed is 64.3 mph. Your odometer will under-count miles by about 7.2%.
Will 235/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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