Tire Size Box

345/35R20 vs 305/35R21

305/35R21 compared with 345/35R20

−0.3% diameter, −1.6" width

305/35R21 is 29.41" tall vs 29.51". Within the 3% rule, so usually interchangeable.

29.5"20" rim4.8"13.6" wide
345/35R20 (solid) compared with 305/35R21 (dashed outline), drawn to scale

With 305/35R21 tires, when your speedometer says 60 mph you are really going

59.8 mph

305/35R21 is −0.10 inches (−0.3%) in overall diameter. That is within the 3% most shops and makers allow.

345/35R20305/35R21Difference%
Overall diameter29.51"29.41"−0.10"−0.3%
Section width13.58"12.01"−1.57"−11.6%
Sidewall height4.75"4.20"−0.55"−11.6%
Circumference92.70"92.38"−0.32"−0.3%
Revolutions per mile683.5685.9+2.4+0.3%

Speedometer difference

If your vehicle came with 345/35R20 and you fit 305/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph19.9 mph−0.1 mph
30 mph29.9 mph−0.1 mph
40 mph39.9 mph−0.1 mph
50 mph49.8 mph−0.2 mph
60 mph59.8 mph−0.2 mph
65 mph64.8 mph−0.2 mph
70 mph69.8 mph−0.2 mph
75 mph74.7 mph−0.3 mph
80 mph79.7 mph−0.3 mph

Each size on its own

Questions

Which is bigger, 345/35R20 or 305/35R21?
345/35R20 is taller by 0.10 inches (29.51" vs 29.41"). 345/35R20 is wider by 1.57 inches.
Can I replace 345/35R20 with 305/35R21?
Usually, yes. The diameter difference is −0.3%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 305/35R21 change my speedometer?
When your speedometer reads 60 mph with 305/35R21 fitted (calibrated for 345/35R20), your real speed is 59.8 mph. Your odometer will over-count miles by about 0.3%.
Will 305/35R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.05 inches of ground clearance at the axle.

Other comparisons