Tire Size Box

305/25R21 vs 305/35R21

305/35R21 compared with 305/25R21

+8.9% diameter, 0.0" width

305/35R21 is 29.41" 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 305/35R21 (dashed outline), drawn to scale

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

65.3 mph

305/35R21 is +2.40 inches (+8.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/25R21305/35R21Difference%
Overall diameter27.00"29.41"+2.40"+8.9%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.00"4.20"+1.20"+40.0%
Circumference84.84"92.38"+7.54"+8.9%
Revolutions per mile746.9685.9−61.0−8.2%

Speedometer difference

If your vehicle came with 305/25R21 and you fit 305/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.4 mph+4.4 mph
60 mph65.3 mph+5.3 mph
65 mph70.8 mph+5.8 mph
70 mph76.2 mph+6.2 mph
75 mph81.7 mph+6.7 mph
80 mph87.1 mph+7.1 mph

Each size on its own

Questions

Which is bigger, 305/25R21 or 305/35R21?
305/35R21 is taller by 2.40 inches (29.41" vs 27.00"). 305/25R21 is the same width.
Can I replace 305/25R21 with 305/35R21?
It is not recommended without checking carefully. The diameter difference is +8.9%, 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 305/35R21 change my speedometer?
When your speedometer reads 60 mph with 305/35R21 fitted (calibrated for 305/25R21), your real speed is 65.3 mph. Your odometer will under-count miles by about 8.9%.
Will 305/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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