Mine was trimmed on the front side of the wheel wells. That's the easy trim since there's no bracket to get in your way. You should be able to mark the cladding with a grease pencil to guide you in how much will have to be cut off or better yet, take a picture of your wheel well & practice on that. There are some wheel well pix in my gallery that I took for someone a while back (they didn't come out very good though).

Can't say how my gas mileage was effected since it was done by the PO.