Tire Size Box

345/40R20 vs 345/45R20

345/45R20 compared with 345/40R20

+4.4% diameter, 0.0" width

345/45R20 is 32.22" tall vs 30.87". More than the 3% rule of thumb, so check carefully before switching.

30.9"20" rim5.4"13.6" wide
345/40R20 (solid) compared with 345/45R20 (dashed outline), drawn to scale

With 345/45R20 tires, when your speedometer says 60 mph you are really going

62.6 mph

345/45R20 is +1.36 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/40R20345/45R20Difference%
Overall diameter30.87"32.22"+1.36"+4.4%
Section width13.58"13.58"0.00"0.0%
Sidewall height5.43"6.11"+0.68"+12.5%
Circumference96.97"101.24"+4.27"+4.4%
Revolutions per mile653.4625.9−27.5−4.2%

Speedometer difference

If your vehicle came with 345/40R20 and you fit 345/45R20, 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 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 345/40R20 or 345/45R20?
345/45R20 is taller by 1.36 inches (32.22" vs 30.87"). 345/40R20 is the same width.
Can I replace 345/40R20 with 345/45R20?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 345/45R20 change my speedometer?
When your speedometer reads 60 mph with 345/45R20 fitted (calibrated for 345/40R20), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.4%.
Will 345/45R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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