Tire Size Box

355/35R21 vs 345/40R21

345/40R21 compared with 355/35R21

+3.5% diameter, −0.4" width

345/40R21 is 31.87" tall vs 30.78". More than the 3% rule of thumb, so check carefully before switching.

30.8"21" rim4.9"14.0" wide
355/35R21 (solid) compared with 345/40R21 (dashed outline), drawn to scale

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

62.1 mph

345/40R21 is +1.08 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/35R21345/40R21Difference%
Overall diameter30.78"31.87"+1.08"+3.5%
Section width13.98"13.58"−0.39"−2.8%
Sidewall height4.89"5.43"+0.54"+11.1%
Circumference96.71"100.11"+3.40"+3.5%
Revolutions per mile655.2632.9−22.3−3.4%

Speedometer difference

If your vehicle came with 355/35R21 and you fit 345/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 355/35R21 or 345/40R21?
345/40R21 is taller by 1.08 inches (31.87" vs 30.78"). 355/35R21 is wider by 0.39 inches.
Can I replace 355/35R21 with 345/40R21?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 355/35R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 345/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

Other comparisons