i get about 16-17 on the highway at about 70 and on city streets i get about 12-13

 

Yea, to calculate, all you need to do is fully fill up, drive however much you want, and fully fill up again. None of that "put $20 in" crap.
But really, you need to fill it up ALL the way -- to where you can see fuel in the fuel filler tube. I find that I can usually add 1.5 gallons from when the nozzle clicks off until its "as full as it gets," but that differs from pump to pump.
(In my Toyota, for example, I can fit another 2 gallons!)

 

thats the same with me, i can usually get another galoon or so, but in my dads stratus if you go over after it clicks it starts to leak out.

 

Well.... My trip meter and travel time matched mapquest to within 5 miles and 10 minutes(Stopped for breakfast on the way down, and dinner on the way back) each way. And those electronic speed signs on the side of the road(They're everywhere near me) tell me that I'm about 1mph fast at 55. I average about 20mpg city, if I'm nice, 16mpg if I'm drivin it like I stole it.
I don't know what to say, man. Before I lost second gear, I would consistently get rubber in the first three gears. 375 miles before the fuel light came on was uncommon.

 

Holy Crap!! 27 MPG with a 318......Really? Even the fact that yours is a 2wd and ours are 4wd doesn't make that kind of difference but, I feel you brother. No one believes me when I tell them that my 1983 Ford Ranger 2.3L 4 speed manual gave me 40 MPG Highway until I took it in for its 5000 mile service when the dealership replaced my carb. The only reason I know the swapped carbs is because on the one I had the mixture screw limiters were blue and broken. When I got the truck back the carb was all shiny new and the limiter caps were red and new. From that point on the truck only gave me 23 - 25 MPG highway. Of course I had no proof so it's my word against theirs.

 

Are you sure your trip meter is really in miles? Are you sure its not KM? Cause 375 km = 233 miles, and that sounds a lot more like our trucks.

Sorry, I'm just very skeptical, cause that seems too unreal. Tell us about the setup you have, cause if you are calculating the accurate milage, I want to know what you have. What tires are you running and what size are they? What rear end gears do you have? What kind of gas do you fill up with? Ect.

 

92blackls. i had a 78 olds cutlass supreme, swapped the 2 barrel carb and dome on the 305 for a Rochester 4 barrel, had to make my own linkage for the carb choke to dome out of coat hanger wire (dome and carb linkage where not the same because of different years for each) and i got at least the mileage you get and i put it to the floor every chance i got. not saying i'm good, just saying you got a serious issue somewhere.

my 89 3.9 4x4 regularly cracked the 20 mpg mark. the only real changes i made where platinum plugs and synthetic fluids in everything, also kept a clean air filter in it.

i know i'm going to get a lot of crap for this but you say Canadian gas is bad, got some bad news for you, i drove from Nova Scotia to Florida and the gas in Canada and New England was the best, the further south i went the more it smelled like diesel and the lower mileage i got. Florida got better but the southeast in general was bad. (slt is going to be all over me for this one) just telling you guys my experience from 1989. if you stop to think of it i get 20 regularly and you guys struggle to get 17 in a 3.9.

 

27 mpg on a 318? TEACH ME OBIE ONE

 

Lol, why would I be all over you for that? You're probably right. I can't speak for how good or bad the gas up north is, but I will agree that it is terrible around here. There used to be only one gas station I would fill my truck up with around here cause for a while they still had 100% gas, but then they finally switched over to e10 and they became no better than any of the other stations around here. I think e10 is crap, but it's all thats available in my area nowadays.

 

You also have to remember that northern territories add additives to prevent gas from freezing as well. The US used an additive that they ended up banning. I cant remember the name of it. But the additive was proven to cure malaria or small pox or some crap like that.

Places like UTAH is 85 octane because of the altitude. Which I have used 85 octane at sea level and the fuel mileage was 2mpg better then using 87. Why they don't use 85 everywhere is beyond me.