Tire Size Box

305/40R20 vs 305/50R20

305/50R20 compared with 305/40R20

+8.1% diameter, 0.0" width

305/50R20 is 32.01" tall vs 29.61". More than the 3% rule of thumb, so check carefully before switching.

29.6"20" rim4.8"12.0" wide
305/40R20 (solid) compared with 305/50R20 (dashed outline), drawn to scale

With 305/50R20 tires, when your speedometer says 60 mph you are really going

64.9 mph

305/50R20 is +2.40 inches (+8.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R20305/50R20Difference%
Overall diameter29.61"32.01"+2.40"+8.1%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"6.00"+1.20"+25.0%
Circumference93.01"100.56"+7.54"+8.1%
Revolutions per mile681.2630.1−51.1−7.5%

Speedometer difference

If your vehicle came with 305/40R20 and you fit 305/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 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, 305/40R20 or 305/50R20?
305/50R20 is taller by 2.40 inches (32.01" vs 29.61"). 305/40R20 is the same width.
Can I replace 305/40R20 with 305/50R20?
It is not recommended without checking carefully. The diameter difference is +8.1%, 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 305/50R20 change my speedometer?
When your speedometer reads 60 mph with 305/50R20 fitted (calibrated for 305/40R20), your real speed is 64.9 mph. Your odometer will under-count miles by about 8.1%.
Will 305/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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