Tire Size Box

305/35R17 vs 305/40R17

305/40R17 compared with 305/35R17

+4.7% diameter, 0.0" width

305/40R17 is 26.61" tall vs 25.41". More than the 3% rule of thumb, so check carefully before switching.

25.4"17" rim4.2"12.0" wide
305/35R17 (solid) compared with 305/40R17 (dashed outline), drawn to scale

With 305/40R17 tires, when your speedometer says 60 mph you are really going

62.8 mph

305/40R17 is +1.20 inches (+4.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/35R17305/40R17Difference%
Overall diameter25.41"26.61"+1.20"+4.7%
Section width12.01"12.01"0.00"0.0%
Sidewall height4.20"4.80"+0.60"+14.3%
Circumference79.81"83.59"+3.77"+4.7%
Revolutions per mile793.8758.0−35.8−4.5%

Speedometer difference

If your vehicle came with 305/35R17 and you fit 305/40R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.9 mph+1.9 mph
50 mph52.4 mph+2.4 mph
60 mph62.8 mph+2.8 mph
65 mph68.1 mph+3.1 mph
70 mph73.3 mph+3.3 mph
75 mph78.5 mph+3.5 mph
80 mph83.8 mph+3.8 mph

Each size on its own

Questions

Which is bigger, 305/35R17 or 305/40R17?
305/40R17 is taller by 1.20 inches (26.61" vs 25.41"). 305/35R17 is the same width.
Can I replace 305/35R17 with 305/40R17?
It is not recommended without checking carefully. The diameter difference is +4.7%, 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 305/40R17 change my speedometer?
When your speedometer reads 60 mph with 305/40R17 fitted (calibrated for 305/35R17), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.7%.
Will 305/40R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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