Tire Size Box

245/40R18 vs 245/50R18

245/50R18 compared with 245/40R18

+7.5% diameter, 0.0" width

245/50R18 is 27.65" tall vs 25.72". More than the 3% rule of thumb, so check carefully before switching.

25.7"18" rim3.9"9.6" wide
245/40R18 (solid) compared with 245/50R18 (dashed outline), drawn to scale

With 245/50R18 tires, when your speedometer says 60 mph you are really going

64.5 mph

245/50R18 is +1.93 inches (+7.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/40R18245/50R18Difference%
Overall diameter25.72"27.65"+1.93"+7.5%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.86"4.82"+0.96"+25.0%
Circumference80.79"86.85"+6.06"+7.5%
Revolutions per mile784.2729.5−54.7−7.0%

Speedometer difference

If your vehicle came with 245/40R18 and you fit 245/50R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.0 mph+3.0 mph
50 mph53.8 mph+3.8 mph
60 mph64.5 mph+4.5 mph
65 mph69.9 mph+4.9 mph
70 mph75.3 mph+5.3 mph
75 mph80.6 mph+5.6 mph
80 mph86.0 mph+6.0 mph

Each size on its own

Questions

Which is bigger, 245/40R18 or 245/50R18?
245/50R18 is taller by 1.93 inches (27.65" vs 25.72"). 245/40R18 is the same width.
Can I replace 245/40R18 with 245/50R18?
It is not recommended without checking carefully. The diameter difference is +7.5%, 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 245/50R18 change my speedometer?
When your speedometer reads 60 mph with 245/50R18 fitted (calibrated for 245/40R18), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.5%.
Will 245/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.96 inches of ground clearance at the axle.

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