Tire Size Box

235/40R17 vs 235/45R17

235/45R17 compared with 235/40R17

+3.8% diameter, 0.0" width

235/45R17 is 25.33" tall vs 24.40". More than the 3% rule of thumb, so check carefully before switching.

24.4"17" rim3.7"9.3" wide
235/40R17 (solid) compared with 235/45R17 (dashed outline), drawn to scale

With 235/45R17 tires, when your speedometer says 60 mph you are really going

62.3 mph

235/45R17 is +0.93 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/40R17235/45R17Difference%
Overall diameter24.40"25.33"+0.93"+3.8%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.70"4.16"+0.46"+12.5%
Circumference76.66"79.57"+2.91"+3.8%
Revolutions per mile826.5796.3−30.2−3.7%

Speedometer difference

If your vehicle came with 235/40R17 and you fit 235/45R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 235/40R17 or 235/45R17?
235/45R17 is taller by 0.93 inches (25.33" vs 24.40"). 235/40R17 is the same width.
Can I replace 235/40R17 with 235/45R17?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 235/45R17 change my speedometer?
When your speedometer reads 60 mph with 235/45R17 fitted (calibrated for 235/40R17), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 235/45R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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