Tire Size Box

245/40R17 vs 245/45R17

245/45R17 compared with 245/40R17

+3.9% diameter, 0.0" width

245/45R17 is 25.68" tall vs 24.72". More than the 3% rule of thumb, so check carefully before switching.

24.7"17" rim3.9"9.6" wide
245/40R17 (solid) compared with 245/45R17 (dashed outline), drawn to scale

With 245/45R17 tires, when your speedometer says 60 mph you are really going

62.3 mph

245/45R17 is +0.96 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/40R17245/45R17Difference%
Overall diameter24.72"25.68"+0.96"+3.9%
Section width9.65"9.65"0.00"0.0%
Sidewall height3.86"4.34"+0.48"+12.5%
Circumference77.65"80.68"+3.03"+3.9%
Revolutions per mile816.0785.3−30.6−3.8%

Speedometer difference

If your vehicle came with 245/40R17 and you fit 245/45R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 245/40R17 or 245/45R17?
245/45R17 is taller by 0.96 inches (25.68" vs 24.72"). 245/40R17 is the same width.
Can I replace 245/40R17 with 245/45R17?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 245/45R17 change my speedometer?
When your speedometer reads 60 mph with 245/45R17 fitted (calibrated for 245/40R17), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 245/45R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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