Tire Size Box

305/40R17 vs 305/50R17

305/50R17 compared with 305/40R17

+9.0% diameter, 0.0" width

305/50R17 is 29.01" tall vs 26.61". More than the 3% rule of thumb, so check carefully before switching.

26.6"17" rim4.8"12.0" wide
305/40R17 (solid) compared with 305/50R17 (dashed outline), drawn to scale

With 305/50R17 tires, when your speedometer says 60 mph you are really going

65.4 mph

305/50R17 is +2.40 inches (+9.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/40R17305/50R17Difference%
Overall diameter26.61"29.01"+2.40"+9.0%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.80"6.00"+1.20"+25.0%
Circumference83.59"91.13"+7.54"+9.0%
Revolutions per mile758.0695.3−62.8−8.3%

Speedometer difference

If your vehicle came with 305/40R17 and you fit 305/50R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.4 mph+5.4 mph
65 mph70.9 mph+5.9 mph
70 mph76.3 mph+6.3 mph
75 mph81.8 mph+6.8 mph
80 mph87.2 mph+7.2 mph

Each size on its own

Questions

Which is bigger, 305/40R17 or 305/50R17?
305/50R17 is taller by 2.40 inches (29.01" vs 26.61"). 305/40R17 is the same width.
Can I replace 305/40R17 with 305/50R17?
It is not recommended without checking carefully. The diameter difference is +9.0%, 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 305/50R17 change my speedometer?
When your speedometer reads 60 mph with 305/50R17 fitted (calibrated for 305/40R17), your real speed is 65.4 mph. Your odometer will under-count miles by about 9.0%.
Will 305/50R17 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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