So I'm reading that my 2008 Mazda3 i Touring is supposed to get 24city/32 highway. So supposing you average ~25miles mpg x 14.5 gallon tank, that equates to 362.5 miles per tank. Well here's the problem - I'm only getting about ~300-320ish miles per tank. I don't speed, nor do an excessive amount of stop-n-go, so I'm pretty clueless as to why the gas mileage is suffering. Any thoughts?

 

Ask yourself this; when was the last time you filled your gas tank with 14.5 gallons of gasoline?

Before you even bother answering I'll tell you; probably never.

The only true way to figure your fuel economy is to add up the miles you've driven over a period of say three to five fill-ups, and then add the number of gallons it took at each fill-up and then divide the miles by the gallons.

 

numbers provided by manufactures like hp and tq rating as well as advertised mpg can and are altered to make the car more appealing to potential customers, also they way achieve the advertised mpg is based more on projected data with some real world testing.

In short you may get less or more than advertised no matter how or where you drive the car

 

Do what "shipo" advised. For a quicker off the cuff faster answer, after a fillup, divide the number of miles driven by the amount of gas you filled up with. Example: 224 miles driven, 9.5 gallons of gas at fillup; 224 divided by 9.5=23.57 miles per gallon. Make it a practice to stop filling your tank the second the pump handle clicks off. Constantly topping off may result in you soaking the fuel vapor recovery emission tank and may cause the "service engine" light to go on. A needless inconvenience.

Good luck,
virg