Tire Size Box

305/35R18 vs 295/40R18

295/40R18 compared with 305/35R18

+3.4% diameter, −0.4" width

295/40R18 is 27.29" tall vs 26.41". More than the 3% rule of thumb, so check carefully before switching.

26.4"18" rim4.2"12.0" wide
305/35R18 (solid) compared with 295/40R18 (dashed outline), drawn to scale

With 295/40R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

295/40R18 is +0.89 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R18295/40R18Difference%
Overall diameter26.41"27.29"+0.89"+3.4%
Section width12.01"11.61"−0.39"−3.3%
Sidewall height4.20"4.65"+0.44"+10.5%
Circumference82.96"85.74"+2.78"+3.4%
Revolutions per mile763.8739.0−24.8−3.2%

Speedometer difference

If your vehicle came with 305/35R18 and you fit 295/40R18, 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, 305/35R18 or 295/40R18?
295/40R18 is taller by 0.89 inches (27.29" vs 26.41"). 305/35R18 is wider by 0.39 inches.
Can I replace 305/35R18 with 295/40R18?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 295/40R18 change my speedometer?
When your speedometer reads 60 mph with 295/40R18 fitted (calibrated for 305/35R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 295/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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