Tire Size Box

305/45R20 vs 305/50R20

305/50R20 compared with 305/45R20

+3.9% diameter, 0.0" width

305/50R20 is 32.01" tall vs 30.81". More than the 3% rule of thumb, so check carefully before switching.

30.8"20" rim5.4"12.0" wide
305/45R20 (solid) compared with 305/50R20 (dashed outline), drawn to scale

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

62.3 mph

305/50R20 is +1.20 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R20305/50R20Difference%
Overall diameter30.81"32.01"+1.20"+3.9%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.00"+0.60"+11.1%
Circumference96.78"100.56"+3.77"+3.9%
Revolutions per mile654.7630.1−24.6−3.8%

Speedometer difference

If your vehicle came with 305/45R20 and you fit 305/50R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 305/45R20 or 305/50R20?
305/50R20 is taller by 1.20 inches (32.01" vs 30.81"). 305/45R20 is the same width.
Can I replace 305/45R20 with 305/50R20?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 305/45R20), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 305/50R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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