Doesn't matter what the tank capacity is -- use the reading off the station pump to determine how much you've used. And you have to be sure your odo is reasonably accurate - changing tire size and/or rear gears without correcting the speedo can really throw the odometer reading off which wreaks havoc with fuel mileage calcs. You can't do a reasonable estimate of mileage by looking at the fuel gauge and estimating fuel use (e.g. - 3/4 of a tank on the gauge means I used about 11 gallons) -- there's WAY too much variation on the gauges.
The biggest variable is driving style and conditions. It's pretty difficult for these engines in 3000-3500 lb. cars to do any better than about 14-15 mpg in heavy stop and go conditions. With a modded car that makes more HP at high rpm, and less torque at low rpm, it can be even worse as the engine spends most of it's time on the street in an rpm range that isn't very efficient for the modded motors. I live in a relatively small town, so my 'city' mileage isn't really impacted by heavy stop and go. With a mild H/C/I combo, and 3.73 gears (with slightly shorter tires than a STang - think 3.90 gears in a Stang) mine consistently gets 19-20 mpg around town, and 25-27 mpg on the highway. That's driven VERY conservatively -- I spend as little time above 2500 rpm as I have to. I'm turning over around 2300 rpm at 65 mph.
but i did gaina bout 2 mpg after but still.....
I almost got 10mpg this tank. I figure with normal driving it should improve. It should get at least 15 or so around town i'm hoping. 
