Tire Size Box

305/30R20 vs 305/40R20

305/40R20 compared with 305/30R20

+8.8% diameter, 0.0" width

305/40R20 is 29.61" tall vs 27.20". More than the 3% rule of thumb, so check carefully before switching.

27.2"20" rim3.6"12.0" wide
305/30R20 (solid) compared with 305/40R20 (dashed outline), drawn to scale

With 305/40R20 tires, when your speedometer says 60 mph you are really going

65.3 mph

305/40R20 is +2.40 inches (+8.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R20305/40R20Difference%
Overall diameter27.20"29.61"+2.40"+8.8%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.80"+1.20"+33.3%
Circumference85.47"93.01"+7.54"+8.8%
Revolutions per mile741.3681.2−60.1−8.1%

Speedometer difference

If your vehicle came with 305/30R20 and you fit 305/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.4 mph+4.4 mph
60 mph65.3 mph+5.3 mph
65 mph70.7 mph+5.7 mph
70 mph76.2 mph+6.2 mph
75 mph81.6 mph+6.6 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 305/30R20 or 305/40R20?
305/40R20 is taller by 2.40 inches (29.61" vs 27.20"). 305/30R20 is the same width.
Can I replace 305/30R20 with 305/40R20?
It is not recommended without checking carefully. The diameter difference is +8.8%, 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 305/40R20 change my speedometer?
When your speedometer reads 60 mph with 305/40R20 fitted (calibrated for 305/30R20), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.8%.
Will 305/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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