Tire Size Box

305/40R21 vs 305/50R21

305/50R21 compared with 305/40R21

+7.8% diameter, 0.0" width

305/50R21 is 33.01" tall vs 30.61". More than the 3% rule of thumb, so check carefully before switching.

30.6"21" rim4.8"12.0" wide
305/40R21 (solid) compared with 305/50R21 (dashed outline), drawn to scale

With 305/50R21 tires, when your speedometer says 60 mph you are really going

64.7 mph

305/50R21 is +2.40 inches (+7.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R21305/50R21Difference%
Overall diameter30.61"33.01"+2.40"+7.8%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"6.00"+1.20"+25.0%
Circumference96.15"103.70"+7.54"+7.8%
Revolutions per mile659.0611.0−47.9−7.3%

Speedometer difference

If your vehicle came with 305/40R21 and you fit 305/50R21, 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 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.3 mph+6.3 mph

Each size on its own

Questions

Which is bigger, 305/40R21 or 305/50R21?
305/50R21 is taller by 2.40 inches (33.01" vs 30.61"). 305/40R21 is the same width.
Can I replace 305/40R21 with 305/50R21?
It is not recommended without checking carefully. The diameter difference is +7.8%, 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 305/50R21 change my speedometer?
When your speedometer reads 60 mph with 305/50R21 fitted (calibrated for 305/40R21), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.8%.
Will 305/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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