When we got together, one of the things my husband taught me was to keep track of my car's information. I keep a small notepad in my driver's side door pocket. Your notepad can be whatever you like - a yellow pad, a small binder, a small spiral - whatever works best for you. Start by writing your mileage that you're at right now. Then, record every oil change and everytime you get gas. This is the way I record my entries.
Each time I get gas, I take the ending mileage - the ending mileage from the gas entry before = the miles I drove since getting gas. Take the # of miles / the # of gals. you purchased this time = your avg mpg. In my example below: 21,500 (my ending mileage at this gas fill-up)-21,200 (my mileage from the previous entry) = 300. 300/18.14 (the # of gals I purchased this time) = 16.54 (average mpg since my last fill-up).
Starting 21,200 Jan. 1, 2011
Gas: Jan. 18, 2011 21,500 (300 mi.)
$2.56/gal 18.14 gals. 16.54 avg.mpg
Oil change 26,200 July 1, 2011
Also keep track of all the other maintenance and repairs you have done. When you're trying hard to remember when you had your last air filter change, you can simply look back and find out if it's time for a new one yet. Writing down this information will help keep you accountable for your vehicle's maintenance and will reward you with a better running car - for longer. And that saves us money!
Some other ways to save on gas: if you have an Android phone or an iPhone, you should be able to download an app that tells you at what gas station (near you) you can find the cheapest gas. I use Poynt. I'm not a gas expert, so you'll have to decide which station you prefer, but there are a few stations whose gas I feel made my car run badly, so, I stay away from them. If you prefer Shell gas, you can get a Shell Saver Card and save $.10 per gallon now through April 30, 2011.