Iphone users grab the Gas Cubby app, free version sponsored by Fram. There are others, but I like this one. Input mx and fuel items each repair/fillup and have a running record of your costs. Last 5 fillups for me were: 18.4, 17.7, 17.7, 17.7, 17.7. Looks like the OP has me beat recently. Since you're putting in mileage each fillup it also gives messages when you need to change oil, air filter, plugs... whatever you put in and the frequency you ask for.
Okay, so maybe I'm a nut about this kind of stuff...
Patrick

 

I use Gas Buddy for my truck. Road trip Montreal - Toronto netted me a best fill of 9.9l/100km = 23.7mpg over 376km all highway. Entire trip including driving around in the city was 10.1l/100km = 23.5mpg.

This weekend did a trip from Montreal to Boston and back and did a bit worse at 10.5l/100km because I upped the speed to about 62mph and the highway has quite a few hills.

Worst tank lifetime for me was 14.9l/100km = 15.7 and really don't want to see any more of those with fuel at $5.50/gallon up here.

 

Oh wow thanks man, downloading it now. Will def get some use out of it.

 

I recently made a trip from school to my grandparents place in PA. It was about a 700mi round trip with a decent amount of highway and some city but not much. I got 19.4 down and 20.1 back. That was the first time I had seen 20+ mpg in my truck. Shes been running really well lately and so I've been getting my gas from the same place the last couple times which is a Gulf gas station. Not sure if it is affecting anything but why not keep doing what works? Plus it's usually the cheapest in town or close to it.

By the way, I calculated mine using the odometer and the amount of gas I used since I don't have the overhead display. I have the 'clip' lol.

 

I did this on a long trip and mine was pretty accurate.. how many miles it told me i had to go till empty was about 3 miles shy of what I actually got..