Tire Size Box

345/35R21 vs 345/40R21

345/40R21 compared with 345/35R21

+4.5% diameter, 0.0" width

345/40R21 is 31.87" 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 345/40R21 (dashed outline), drawn to scale

With 345/40R21 tires, when your speedometer says 60 mph you are really going

62.7 mph

345/40R21 is +1.36 inches (+4.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/35R21345/40R21Difference%
Overall diameter30.51"31.87"+1.36"+4.5%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.75"5.43"+0.68"+14.3%
Circumference95.84"100.11"+4.27"+4.5%
Revolutions per mile661.1632.9−28.2−4.3%

Speedometer difference

If your vehicle came with 345/35R21 and you fit 345/40R21, 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 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 345/35R21 or 345/40R21?
345/40R21 is taller by 1.36 inches (31.87" vs 30.51"). 345/35R21 is the same width.
Can I replace 345/35R21 with 345/40R21?
It is not recommended without checking carefully. The diameter difference is +4.5%, 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 345/40R21 change my speedometer?
When your speedometer reads 60 mph with 345/40R21 fitted (calibrated for 345/35R21), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.5%.
Will 345/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.68 inches of ground clearance at the axle.

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