Tire Size Box

255/30R20 vs 255/40R20

255/40R20 compared with 255/30R20

+7.7% diameter, 0.0" width

255/40R20 is 28.03" tall vs 26.02". More than the 3% rule of thumb, so check carefully before switching.

26.0"20" rim3.0"10.0" wide
255/30R20 (solid) compared with 255/40R20 (dashed outline), drawn to scale

With 255/40R20 tires, when your speedometer says 60 mph you are really going

64.6 mph

255/40R20 is +2.01 inches (+7.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/30R20255/40R20Difference%
Overall diameter26.02"28.03"+2.01"+7.7%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.01"4.02"+1.00"+33.3%
Circumference81.76"88.06"+6.31"+7.7%
Revolutions per mile775.0719.5−55.5−7.2%

Speedometer difference

If your vehicle came with 255/30R20 and you fit 255/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.6 mph+4.6 mph
65 mph70.0 mph+5.0 mph
70 mph75.4 mph+5.4 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 255/30R20 or 255/40R20?
255/40R20 is taller by 2.01 inches (28.03" vs 26.02"). 255/30R20 is the same width.
Can I replace 255/30R20 with 255/40R20?
It is not recommended without checking carefully. The diameter difference is +7.7%, 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 255/40R20 change my speedometer?
When your speedometer reads 60 mph with 255/40R20 fitted (calibrated for 255/30R20), your real speed is 64.6 mph. Your odometer will under-count miles by about 7.7%.
Will 255/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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