Tire Size Box

265/40R18 vs 265/50R18

265/50R18 compared with 265/40R18

+7.9% diameter, 0.0" width

265/50R18 is 28.43" tall vs 26.35". More than the 3% rule of thumb, so check carefully before switching.

26.3"18" rim4.2"10.4" wide
265/40R18 (solid) compared with 265/50R18 (dashed outline), drawn to scale

With 265/50R18 tires, when your speedometer says 60 mph you are really going

64.8 mph

265/50R18 is +2.09 inches (+7.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

265/40R18265/50R18Difference%
Overall diameter26.35"28.43"+2.09"+7.9%
Section width10.43"10.43"0.00"0.0%
Sidewall height4.17"5.22"+1.04"+25.0%
Circumference82.77"89.33"+6.56"+7.9%
Revolutions per mile765.5709.3−56.2−7.3%

Speedometer difference

If your vehicle came with 265/40R18 and you fit 265/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.2 mph+3.2 mph
50 mph54.0 mph+4.0 mph
60 mph64.8 mph+4.8 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.3 mph+6.3 mph

Each size on its own

Questions

Which is bigger, 265/40R18 or 265/50R18?
265/50R18 is taller by 2.09 inches (28.43" vs 26.35"). 265/40R18 is the same width.
Can I replace 265/40R18 with 265/50R18?
It is not recommended without checking carefully. The diameter difference is +7.9%, 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 265/50R18 change my speedometer?
When your speedometer reads 60 mph with 265/50R18 fitted (calibrated for 265/40R18), your real speed is 64.8 mph. Your odometer will under-count miles by about 7.9%.
Will 265/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.04 inches of ground clearance at the axle.

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