Tire Size Box

235/45R18 vs 235/50R18

235/50R18 compared with 235/45R18

+3.5% diameter, 0.0" width

235/50R18 is 27.25" tall vs 26.33". More than the 3% rule of thumb, so check carefully before switching.

26.3"18" rim4.2"9.3" wide
235/45R18 (solid) compared with 235/50R18 (dashed outline), drawn to scale

With 235/50R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

235/50R18 is +0.93 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/45R18235/50R18Difference%
Overall diameter26.33"27.25"+0.93"+3.5%
Section width9.25"9.25"0.00"0.0%
Sidewall height4.16"4.63"+0.46"+11.1%
Circumference82.71"85.61"+2.91"+3.5%
Revolutions per mile766.1740.1−26.0−3.4%

Speedometer difference

If your vehicle came with 235/45R18 and you fit 235/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 235/45R18 or 235/50R18?
235/50R18 is taller by 0.93 inches (27.25" vs 26.33"). 235/45R18 is the same width.
Can I replace 235/45R18 with 235/50R18?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 235/50R18 change my speedometer?
When your speedometer reads 60 mph with 235/50R18 fitted (calibrated for 235/45R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 235/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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