you may say i’m crazy but i keep a log of every single fill-up. here are some stats:
best gas and what I regularly use: 91 Sunoco
A lot of 240’s have the “check engine” light issue. It’s usually related to the knock sensor or the oxygen sensor. I had the check engine light about three yrs ago and nissan dealer told me I would have to change the knock sensor. I didn’t want to pay the $350 and since it’s not harmful anyways, I just left it. I was trying gas from different gas stations to see if different gas actually effects gas mileage. I found out that it does. But I also found out that using 91 Sunoco makes the check engine light disappear. I’ve been testing this for about 3 yrs now, and believe me everytime I convince myself that it’s not the gas that’s triggering the check engine light and goto esso, pc or shell, the check engine light comes back. A friend of mine with an auto 240, '95 LE also had the same problem. As soon as he put 91 sunoco, the light went away.
now, the light comes and goes every once in a while. It’ll come back for like 5 days but then disappear for 3 months. so i’m not saying sunoco gas fixed the problem, it just gives me great gas mileage and doesn’t show me the stupid check engine light… i will get the knock sensor fixed someday 
Max fill-up: 65 litres
avg fill-up: 25 litres
I usually only put 25 litres of gas. This increases the gas mileage. Also, I have no spare tire, no jack or any extra crap in my trunk. I take great care of my car and usually can figure out the problem before they occur. worse case scnerio if I get a flat on the hwy (slim chances), I’ll spend the $70 to get it towed, but that’s nothing compared to the fuel cost i would’ve saved. for long drives i do put the spare back in.
After the faded gas light comes on, my car drives for exactly 125 more KM. After the light is solid, it’ll go for about 55KM.
Max KM: 695 (mix hwy & city)
avg KM: 625
best gas mileage: 33 MPG (8.5ltr/100KM?) (back and forth from montreal, with a lot of luggage in the trunk)
avg gas mileage: 25 MPG (10ltr/100KM i think)
lately I’ve been getting aroun 22-23MPG but I know it’s time to replace the fuel filter.
Don’t worry about the needle. The first quarter of the tank will take forever to move the needle. right after that, the needle will drop to a little below half and that’ll take it’s sweet ass time. From what ppl have told me, an avg 240 should run about 525 on a full tank.
first quarter odo will read - 225KM
half odo will read - 350KM
last quarter odo will read - 475KM
at empty odo will read - 525KM
that’s average… i usually get 250, 375, 500, 625
I have more logs of what oil is good, which oil filter, what not to put in your gas tank, etc… just ask what you need to know and I might already have tracked it or currently working on it 
I apologize my post went a little off topic but since this was a discussion realted to gas, I thought I threw in my crap… I know most of you will be like “who the hell does this guys think he is!”… oh well :-p