Tire Size Box

305/45R18 vs 305/50R18

305/50R18 compared with 305/45R18

+4.2% diameter, 0.0" width

305/50R18 is 30.01" tall vs 28.81". More than the 3% rule of thumb, so check carefully before switching.

28.8"18" rim5.4"12.0" wide
305/45R18 (solid) compared with 305/50R18 (dashed outline), drawn to scale

With 305/50R18 tires, when your speedometer says 60 mph you are really going

62.5 mph

305/50R18 is +1.20 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R18305/50R18Difference%
Overall diameter28.81"30.01"+1.20"+4.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.00"+0.60"+11.1%
Circumference90.50"94.27"+3.77"+4.2%
Revolutions per mile700.1672.1−28.0−4.0%

Speedometer difference

If your vehicle came with 305/45R18 and you fit 305/50R18, 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 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 305/45R18 or 305/50R18?
305/50R18 is taller by 1.20 inches (30.01" vs 28.81"). 305/45R18 is the same width.
Can I replace 305/45R18 with 305/50R18?
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 305/50R18 change my speedometer?
When your speedometer reads 60 mph with 305/50R18 fitted (calibrated for 305/45R18), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 305/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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