Tire Size Box

305/35R20 vs 305/40R20

305/40R20 compared with 305/35R20

+4.2% diameter, 0.0" width

305/40R20 is 29.61" tall vs 28.41". More than the 3% rule of thumb, so check carefully before switching.

28.4"20" rim4.2"12.0" wide
305/35R20 (solid) compared with 305/40R20 (dashed outline), drawn to scale

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

62.5 mph

305/40R20 is +1.20 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R20305/40R20Difference%
Overall diameter28.41"29.61"+1.20"+4.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"4.80"+0.60"+14.3%
Circumference89.24"93.01"+3.77"+4.2%
Revolutions per mile710.0681.2−28.8−4.1%

Speedometer difference

If your vehicle came with 305/35R20 and you fit 305/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 305/35R20 or 305/40R20?
305/40R20 is taller by 1.20 inches (29.61" vs 28.41"). 305/35R20 is the same width.
Can I replace 305/35R20 with 305/40R20?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 305/40R20 change my speedometer?
When your speedometer reads 60 mph with 305/40R20 fitted (calibrated for 305/35R20), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 305/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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