Tire Size Box

345/40R21 vs 345/50R21

345/50R21 compared with 345/40R21

+8.5% diameter, 0.0" width

345/50R21 is 34.58" tall vs 31.87". More than the 3% rule of thumb, so check carefully before switching.

31.9"21" rim5.4"13.6" wide
345/40R21 (solid) compared with 345/50R21 (dashed outline), drawn to scale

With 345/50R21 tires, when your speedometer says 60 mph you are really going

65.1 mph

345/50R21 is +2.72 inches (+8.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/40R21345/50R21Difference%
Overall diameter31.87"34.58"+2.72"+8.5%
Section width13.58"13.58"0.00"0.0%
Sidewall height5.43"6.79"+1.36"+25.0%
Circumference100.11"108.64"+8.53"+8.5%
Revolutions per mile632.9583.2−49.7−7.9%

Speedometer difference

If your vehicle came with 345/40R21 and you fit 345/50R21, 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 mph43.4 mph+3.4 mph
50 mph54.3 mph+4.3 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph76.0 mph+6.0 mph
75 mph81.4 mph+6.4 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 345/40R21 or 345/50R21?
345/50R21 is taller by 2.72 inches (34.58" vs 31.87"). 345/40R21 is the same width.
Can I replace 345/40R21 with 345/50R21?
It is not recommended without checking carefully. The diameter difference is +8.5%, 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 345/50R21 change my speedometer?
When your speedometer reads 60 mph with 345/50R21 fitted (calibrated for 345/40R21), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.5%.
Will 345/50R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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