Tire Size Box

235/40R18 vs 235/50R18

235/50R18 compared with 235/40R18

+7.3% diameter, 0.0" width

235/50R18 is 27.25" tall vs 25.40". More than the 3% rule of thumb, so check carefully before switching.

25.4"18" rim3.7"9.3" wide
235/40R18 (solid) compared with 235/50R18 (dashed outline), drawn to scale

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

64.4 mph

235/50R18 is +1.85 inches (+7.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R18235/50R18Difference%
Overall diameter25.40"27.25"+1.85"+7.3%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.63"+0.93"+25.0%
Circumference79.80"85.61"+5.81"+7.3%
Revolutions per mile794.0740.1−53.9−6.8%

Speedometer difference

If your vehicle came with 235/40R18 and you fit 235/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.2 mph+2.2 mph
40 mph42.9 mph+2.9 mph
50 mph53.6 mph+3.6 mph
60 mph64.4 mph+4.4 mph
65 mph69.7 mph+4.7 mph
70 mph75.1 mph+5.1 mph
75 mph80.5 mph+5.5 mph
80 mph85.8 mph+5.8 mph

Each size on its own

Questions

Which is bigger, 235/40R18 or 235/50R18?
235/50R18 is taller by 1.85 inches (27.25" vs 25.40"). 235/40R18 is the same width.
Can I replace 235/40R18 with 235/50R18?
It is not recommended without checking carefully. The diameter difference is +7.3%, 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 235/40R18), your real speed is 64.4 mph. Your odometer will under-count miles by about 7.3%.
Will 235/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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