Tire Size Box

345/35R21 vs 345/45R21

345/45R21 compared with 345/35R21

+8.9% diameter, 0.0" width

345/45R21 is 33.22" tall vs 30.51". More than the 3% rule of thumb, so check carefully before switching.

30.5"21" rim4.8"13.6" wide
345/35R21 (solid) compared with 345/45R21 (dashed outline), drawn to scale

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

65.3 mph

345/45R21 is +2.72 inches (+8.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/35R21345/45R21Difference%
Overall diameter30.51"33.22"+2.72"+8.9%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.75"6.11"+1.36"+28.6%
Circumference95.84"104.38"+8.53"+8.9%
Revolutions per mile661.1607.0−54.1−8.2%

Speedometer difference

If your vehicle came with 345/35R21 and you fit 345/45R21, 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 mph65.3 mph+5.3 mph
65 mph70.8 mph+5.8 mph
70 mph76.2 mph+6.2 mph
75 mph81.7 mph+6.7 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 345/35R21 or 345/45R21?
345/45R21 is taller by 2.72 inches (33.22" vs 30.51"). 345/35R21 is the same width.
Can I replace 345/35R21 with 345/45R21?
It is not recommended without checking carefully. The diameter difference is +8.9%, 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 345/35R21), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.9%.
Will 345/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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