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I have 315/35 17s on back curently with a diameter of 25.79, and if i was to go to a 325/50 15 with a diameter of 27.99 how much would my speedometer be off by? Also what is the backspacing i need for 15 inch rims?
Tire diameter
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Circumference is pi times diameter. 3.14 x 25.79 = 80.98 inches. 3.14 x 27.99 = 87.89 inches. The circumference of the 315/50/15 tire is 8.5% larger so you will actually be travelling 8.5% faster than your speedometer reads. So to go 70 on the interstate you have to drive 64.5. To go 55 you have to drive 50.7.
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