Tire Size Box

305/45R21 vs 305/50R21

305/50R21 compared with 305/45R21

+3.8% diameter, 0.0" width

305/50R21 is 33.01" tall vs 31.81". More than the 3% rule of thumb, so check carefully before switching.

31.8"21" rim5.4"12.0" wide
305/45R21 (solid) compared with 305/50R21 (dashed outline), drawn to scale

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

62.3 mph

305/50R21 is +1.20 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R21305/50R21Difference%
Overall diameter31.81"33.01"+1.20"+3.8%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.00"+0.60"+11.1%
Circumference99.92"103.70"+3.77"+3.8%
Revolutions per mile634.1611.0−23.1−3.6%

Speedometer difference

If your vehicle came with 305/45R21 and you fit 305/50R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 305/45R21 or 305/50R21?
305/50R21 is taller by 1.20 inches (33.01" vs 31.81"). 305/45R21 is the same width.
Can I replace 305/45R21 with 305/50R21?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 305/45R21), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 305/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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