Tire Size Box

345/30R24 vs 345/40R24

345/40R24 compared with 345/30R24

+8.4% diameter, 0.0" width

345/40R24 is 34.87" tall vs 32.15". More than the 3% rule of thumb, so check carefully before switching.

32.1"24" rim4.1"13.6" wide
345/30R24 (solid) compared with 345/40R24 (dashed outline), drawn to scale

With 345/40R24 tires, when your speedometer says 60 mph you are really going

65.1 mph

345/40R24 is +2.72 inches (+8.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/30R24345/40R24Difference%
Overall diameter32.15"34.87"+2.72"+8.4%
Section width13.58"13.58"0.00"0.0%
Sidewall height4.07"5.43"+1.36"+33.3%
Circumference101.00"109.54"+8.53"+8.4%
Revolutions per mile627.3578.4−48.9−7.8%

Speedometer difference

If your vehicle came with 345/30R24 and you fit 345/40R24, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.4 mph+3.4 mph
50 mph54.2 mph+4.2 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph75.9 mph+5.9 mph
75 mph81.3 mph+6.3 mph
80 mph86.8 mph+6.8 mph

Each size on its own

Questions

Which is bigger, 345/30R24 or 345/40R24?
345/40R24 is taller by 2.72 inches (34.87" vs 32.15"). 345/30R24 is the same width.
Can I replace 345/30R24 with 345/40R24?
It is not recommended without checking carefully. The diameter difference is +8.4%, 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 345/40R24 change my speedometer?
When your speedometer reads 60 mph with 345/40R24 fitted (calibrated for 345/30R24), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.4%.
Will 345/40R24 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

Other comparisons