Tire Size Box

345/40R21 vs 345/45R21

345/45R21 compared with 345/40R21

+4.3% diameter, 0.0" width

345/45R21 is 33.22" 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 345/45R21 (dashed outline), drawn to scale

With 345/45R21 tires, when your speedometer says 60 mph you are really going

62.6 mph

345/45R21 is +1.36 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/40R21345/45R21Difference%
Overall diameter31.87"33.22"+1.36"+4.3%
Section width13.58"13.58"0.00"0.0%
Sidewall height5.43"6.11"+0.68"+12.5%
Circumference100.11"104.38"+4.27"+4.3%
Revolutions per mile632.9607.0−25.9−4.1%

Speedometer difference

If your vehicle came with 345/40R21 and you fit 345/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 345/40R21 or 345/45R21?
345/45R21 is taller by 1.36 inches (33.22" vs 31.87"). 345/40R21 is the same width.
Can I replace 345/40R21 with 345/45R21?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 345/45R21 change my speedometer?
When your speedometer reads 60 mph with 345/45R21 fitted (calibrated for 345/40R21), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 345/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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