No savings in gas, and a bad way to fuck up a decent new car.
Best way to get better gas, is lighten the foot on your driving habits, put lighter wheels and tires on the car, and keep your maintenance on the tune-up's up to par.
My 07 Outback got around 28mpg constantly stock, until I put one kayak on the roof, it dropped to 23, and I have been extending the oil changes to 6000 miles...it dropped down to 20. I tossed on another kayak and it dropped to 18mpg. I could easily take the kayaks off, the roof rack off, and do a tune up and expect 28 mpg's again. When I was driving it in MA, the hills got me around 32mpg.
All that on the stock wheels and tires. If I had the bank, I would toss on some Volk wheels and lighter tires...and would expect a little better MPG....but the extra price for expensive wheels and tires would never be offset for the few dollars saved at the pump.
Payments on the car was nothing as long as you had a decent down payment. My subby is now paid off, and I got the title about 2 months early...and right when it hit 100K's.