Tire Size Box

295/30R20 vs 295/40R20

295/40R20 compared with 295/30R20

+8.6% diameter, 0.0" width

295/40R20 is 29.29" tall vs 26.97". More than the 3% rule of thumb, so check carefully before switching.

27.0"20" rim3.5"11.6" wide
295/30R20 (solid) compared with 295/40R20 (dashed outline), drawn to scale

With 295/40R20 tires, when your speedometer says 60 mph you are really going

65.2 mph

295/40R20 is +2.32 inches (+8.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

295/30R20295/40R20Difference%
Overall diameter26.97"29.29"+2.32"+8.6%
Section width11.61"11.61"0.00"0.0%
Sidewall height3.48"4.65"+1.16"+33.3%
Circumference84.72"92.02"+7.30"+8.6%
Revolutions per mile747.8688.5−59.3−7.9%

Speedometer difference

If your vehicle came with 295/30R20 and you fit 295/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.2 mph+5.2 mph
65 mph70.6 mph+5.6 mph
70 mph76.0 mph+6.0 mph
75 mph81.5 mph+6.5 mph
80 mph86.9 mph+6.9 mph

Each size on its own

Questions

Which is bigger, 295/30R20 or 295/40R20?
295/40R20 is taller by 2.32 inches (29.29" vs 26.97"). 295/30R20 is the same width.
Can I replace 295/30R20 with 295/40R20?
It is not recommended without checking carefully. The diameter difference is +8.6%, 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 295/40R20 change my speedometer?
When your speedometer reads 60 mph with 295/40R20 fitted (calibrated for 295/30R20), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.6%.
Will 295/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.16 inches of ground clearance at the axle.

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