Tire Size Box

305/40R18 vs 305/50R18

305/50R18 compared with 305/40R18

+8.7% diameter, 0.0" width

305/50R18 is 30.01" tall vs 27.61". More than the 3% rule of thumb, so check carefully before switching.

27.6"18" rim4.8"12.0" wide
305/40R18 (solid) compared with 305/50R18 (dashed outline), drawn to scale

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

65.2 mph

305/50R18 is +2.40 inches (+8.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R18305/50R18Difference%
Overall diameter27.61"30.01"+2.40"+8.7%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"6.00"+1.20"+25.0%
Circumference86.73"94.27"+7.54"+8.7%
Revolutions per mile730.6672.1−58.5−8.0%

Speedometer difference

If your vehicle came with 305/40R18 and you fit 305/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.3 mph+4.3 mph
60 mph65.2 mph+5.2 mph
65 mph70.7 mph+5.7 mph
70 mph76.1 mph+6.1 mph
75 mph81.5 mph+6.5 mph
80 mph87.0 mph+7.0 mph

Each size on its own

Questions

Which is bigger, 305/40R18 or 305/50R18?
305/50R18 is taller by 2.40 inches (30.01" vs 27.61"). 305/40R18 is the same width.
Can I replace 305/40R18 with 305/50R18?
It is not recommended without checking carefully. The diameter difference is +8.7%, 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 305/40R18), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.7%.
Will 305/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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