Tire Size Box

305/35R21 vs 295/40R21

295/40R21 compared with 305/35R21

+3.0% diameter, −0.4" width

295/40R21 is 30.29" 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 295/40R21 (dashed outline), drawn to scale

With 295/40R21 tires, when your speedometer says 60 mph you are really going

61.8 mph

295/40R21 is +0.89 inches (+3.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R21295/40R21Difference%
Overall diameter29.41"30.29"+0.89"+3.0%
Section width12.01"11.61"−0.39"−3.3%
Sidewall height4.20"4.65"+0.44"+10.5%
Circumference92.38"95.16"+2.78"+3.0%
Revolutions per mile685.9665.8−20.1−2.9%

Speedometer difference

If your vehicle came with 305/35R21 and you fit 295/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 305/35R21 or 295/40R21?
295/40R21 is taller by 0.89 inches (30.29" vs 29.41"). 305/35R21 is wider by 0.39 inches.
Can I replace 305/35R21 with 295/40R21?
It is not recommended without checking carefully. The diameter difference is +3.0%, 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 295/40R21 change my speedometer?
When your speedometer reads 60 mph with 295/40R21 fitted (calibrated for 305/35R21), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.0%.
Will 295/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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