Tire Size Box

275/30R20 vs 275/40R20

275/40R20 compared with 275/30R20

+8.2% diameter, 0.0" width

275/40R20 is 28.66" tall vs 26.50". More than the 3% rule of thumb, so check carefully before switching.

26.5"20" rim3.2"10.8" wide
275/30R20 (solid) compared with 275/40R20 (dashed outline), drawn to scale

With 275/40R20 tires, when your speedometer says 60 mph you are really going

64.9 mph

275/40R20 is +2.17 inches (+8.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/30R20275/40R20Difference%
Overall diameter26.50"28.66"+2.17"+8.2%
Section width10.83"10.83"0.00"0.0%
Sidewall height3.25"4.33"+1.08"+33.3%
Circumference83.24"90.04"+6.80"+8.2%
Revolutions per mile761.2703.7−57.5−7.6%

Speedometer difference

If your vehicle came with 275/30R20 and you fit 275/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.1 mph+4.1 mph
60 mph64.9 mph+4.9 mph
65 mph70.3 mph+5.3 mph
70 mph75.7 mph+5.7 mph
75 mph81.1 mph+6.1 mph
80 mph86.5 mph+6.5 mph

Each size on its own

Questions

Which is bigger, 275/30R20 or 275/40R20?
275/40R20 is taller by 2.17 inches (28.66" vs 26.50"). 275/30R20 is the same width.
Can I replace 275/30R20 with 275/40R20?
It is not recommended without checking carefully. The diameter difference is +8.2%, 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 275/40R20 change my speedometer?
When your speedometer reads 60 mph with 275/40R20 fitted (calibrated for 275/30R20), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.2%.
Will 275/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.08 inches of ground clearance at the axle.

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