Tire Size Box

305/25R21 vs 295/30R21

295/30R21 compared with 305/25R21

+3.6% diameter, −0.4" width

295/30R21 is 27.97" tall vs 27.00". More than the 3% rule of thumb, so check carefully before switching.

27.0"21" rim3.0"12.0" wide
305/25R21 (solid) compared with 295/30R21 (dashed outline), drawn to scale

With 295/30R21 tires, when your speedometer says 60 mph you are really going

62.1 mph

295/30R21 is +0.96 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/25R21295/30R21Difference%
Overall diameter27.00"27.97"+0.96"+3.6%
Section width12.01"11.61"−0.39"−3.3%
Sidewall height3.00"3.48"+0.48"+16.1%
Circumference84.84"87.87"+3.03"+3.6%
Revolutions per mile746.9721.1−25.8−3.4%

Speedometer difference

If your vehicle came with 305/25R21 and you fit 295/30R21, 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 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 305/25R21 or 295/30R21?
295/30R21 is taller by 0.96 inches (27.97" vs 27.00"). 305/25R21 is wider by 0.39 inches.
Can I replace 305/25R21 with 295/30R21?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 295/30R21 change my speedometer?
When your speedometer reads 60 mph with 295/30R21 fitted (calibrated for 305/25R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 295/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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