Tire Size Box

315/25R24 vs 305/30R24

305/30R24 compared with 315/25R24

+3.3% diameter, −0.4" width

305/30R24 is 31.20" tall vs 30.20". More than the 3% rule of thumb, so check carefully before switching.

30.2"24" rim3.1"12.4" wide
315/25R24 (solid) compared with 305/30R24 (dashed outline), drawn to scale

With 305/30R24 tires, when your speedometer says 60 mph you are really going

62.0 mph

305/30R24 is +1.00 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R24305/30R24Difference%
Overall diameter30.20"31.20"+1.00"+3.3%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height3.10"3.60"+0.50"+16.2%
Circumference94.88"98.03"+3.15"+3.3%
Revolutions per mile667.8646.3−21.5−3.2%

Speedometer difference

If your vehicle came with 315/25R24 and you fit 305/30R24, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 315/25R24 or 305/30R24?
305/30R24 is taller by 1.00 inches (31.20" vs 30.20"). 315/25R24 is wider by 0.39 inches.
Can I replace 315/25R24 with 305/30R24?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 305/30R24 change my speedometer?
When your speedometer reads 60 mph with 305/30R24 fitted (calibrated for 315/25R24), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 305/30R24 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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