Tire Size Box

305/35R21 vs 305/40R21

305/40R21 compared with 305/35R21

+4.1% diameter, 0.0" width

305/40R21 is 30.61" tall vs 29.41". More than the 3% rule of thumb, so check carefully before switching.

29.4"21" rim4.2"12.0" wide
305/35R21 (solid) compared with 305/40R21 (dashed outline), drawn to scale

With 305/40R21 tires, when your speedometer says 60 mph you are really going

62.5 mph

305/40R21 is +1.20 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R21305/40R21Difference%
Overall diameter29.41"30.61"+1.20"+4.1%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"4.80"+0.60"+14.3%
Circumference92.38"96.15"+3.77"+4.1%
Revolutions per mile685.9659.0−26.9−3.9%

Speedometer difference

If your vehicle came with 305/35R21 and you fit 305/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 305/35R21 or 305/40R21?
305/40R21 is taller by 1.20 inches (30.61" vs 29.41"). 305/35R21 is the same width.
Can I replace 305/35R21 with 305/40R21?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 305/40R21 change my speedometer?
When your speedometer reads 60 mph with 305/40R21 fitted (calibrated for 305/35R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.1%.
Will 305/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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