Can I use regular unleaded fuel with my 2009 Nissan Maxima, and if so how many mpg's would I get.?

It says for maximum performance to use premium fuel right on the fuel door. Low quality regular gasoline will only cause spark knock and ping. In turn ping and knock causes the knock sensor to signal the engine control to retard the timing reducing the power output and adversely effect the gas mileage. So over a 20 gallon tank at 30 more cents a gallon you get 2 to 3 less miles per gallon you'll find out there is no real cost advantage and chance of accelerated engine wear. The gas mileage on the window sticker is very close to what you can actually get using the good fuel.