The difference between 18 x 3.5 and 16 x 3 wheels
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Before (Stock 2002 FLSTCI)

After (Same Bike)

(Shown with PM Villain Wheels, Pulley and Rotor. 
Custom Paint By Color Works-Keller, TX)
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18 x 3.5 can replace your stock 16 x 3 with No modifications and for the same price. Comes ready to bolt on your bike. 2000 and higher models can use a 150 tire set-up that replaces Harley's Wide Tire Kit. '99 and lower must use Harley's Wide tire kit to use the 150. The 18" tire is  low profile and the rim is 2 inches taller to allow it to sit and fit the same as stock.