Ok first the silly answer, then a more serious one Where did you find 17 foot wide tires? ' is foot " is inches wind resistance alone on a 40 foot wide car will be terrible. However, if you mean 17 inch Diameter tires that are wider than stock, they will have two bad effects 1) any new tire gets worse MPG for the first 3000 miles while it wears in 2 to 5 MPG 2) wider tires will have more rolling resistance, 2 to 5 MPG more.
17' wide tires.. mess up MPG? ... they will also mess up your car, and any other car trying to pass you.
You asked for it.... Why don't you go about this properly and list the exact size of wheel (diameter and width) and then list the exact size tire you are referring to (diameter, width, aspect ratio). To save you some effort, Jimbo already answered your question in post #2.
