you should create an account on www.fuelly.com and record it every time you fill up. It works pretty well tracking and is very easy to use.

You can also compare yourself against other models/years/etc or research your next car there as well! (for fuel efficiency)

 

UPDATE ON FUEL MILEAGE

Purchased October 18th 2014.
Today it has 9955 miles and a full tank.

254.6 gallons diesel purchased at $570.34.
Winter tires 7000 miles
95% highway miles
33% 65 to 85 mph to Florida.
62 % 45 to 65 mph to work.
5% city.
Last tank @ $2.57 gal.

That is 39.1 MPG

 

We have a 2015 SE purchased on 14 March 2015. Using aCar, an Android auto tracking app that was purchased by Fuelly, we are averaging 39.543 MPG over the first 3,000 miles. Those are "mixed" miles, but since we are suburban, most miles are highway. Last tank was $2.349 / gal.

 

Overall average on our 2015 6speed is around 41 mpg. I did a 1300 mile road trip last weekend and got 51.9 on the first tank. (calculated. Trip computer was reading 57.2) 95% of the miles were 65-70 mph interstate with the AC on. The car now has 10000 miles on it.

 

I haven't run a trip longer than 100 miles, but on the longer trips, my 2015 6-speed was getting around 44 mpg. Also, those numbers are achievable on trips around town in routes that allow me go around 45 mph for about twelve miles or so. I have right under 8K miles on the odometer, so I figure those are really good numbers being that do not try to drive like I have an uncovered pot of caldo (Mexcian soup) in the trunk. My fuelly numbers state an average of 34mpg. My driving is 80% city.