Tire Size Box

305/30R24 vs 305/40R24

305/40R24 compared with 305/30R24

+7.7% diameter, 0.0" width

305/40R24 is 33.61" tall vs 31.20". More than the 3% rule of thumb, so check carefully before switching.

31.2"24" rim3.6"12.0" wide
305/30R24 (solid) compared with 305/40R24 (dashed outline), drawn to scale

With 305/40R24 tires, when your speedometer says 60 mph you are really going

64.6 mph

305/40R24 is +2.40 inches (+7.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R24305/40R24Difference%
Overall diameter31.20"33.61"+2.40"+7.7%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.80"+1.20"+33.3%
Circumference98.03"105.58"+7.54"+7.7%
Revolutions per mile646.3600.1−46.2−7.1%

Speedometer difference

If your vehicle came with 305/30R24 and you fit 305/40R24, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.8 mph+3.8 mph
60 mph64.6 mph+4.6 mph
65 mph70.0 mph+5.0 mph
70 mph75.4 mph+5.4 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 305/30R24 or 305/40R24?
305/40R24 is taller by 2.40 inches (33.61" vs 31.20"). 305/30R24 is the same width.
Can I replace 305/30R24 with 305/40R24?
It is not recommended without checking carefully. The diameter difference is +7.7%, 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 305/40R24 change my speedometer?
When your speedometer reads 60 mph with 305/40R24 fitted (calibrated for 305/30R24), your real speed is 64.6 mph. Your odometer will under-count miles by about 7.7%.
Will 305/40R24 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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