
first of all, you might want to consider measuring "miles per gallon" as opposed to "miles per tank"... miles per tank is a guestimate at best...miles per gallon is a very useful figure to go buy...no one has the same or even similar "miles to a tank" measurements, because not everyone refills with the same amount of gas left...not all gas gauges are accurate, and you may have 3 gallons left when it reads "E", and the next guy may have only one gallon left when it reads "E"... Now, having said that, this is how to get a Useful idea of how economical your mustang is.... fill your tank up and either A) reset your trip odometer or B) write down your mileage. then, drive normally, and when the tank gets near empty, fill up again. write down the exact amount of fuel it took to refill the tank (I.E., 10.272 gallons), and the exact amount of miles driven since you last filled the tank (ie 165 miles). Divide the number of miles driven by the amount of fuel used and BINGO, you get 16.06 miles per gallon, a useful figure to work with...
Now, on to suggestions to improve MPG....
a full tune up> new spark plugs, wires, distributor cap and rotor, fuel filter, pcv, etc.
K&N air filter
inflate tires to Maximum psi listed on the side of the tire
underdrive pulleys improve mileage
synthetic lubricants throughout the car are good for 1 or 2 mpg
bad o2 sensors KILL gas mileage
also, lose as much weight as you can (from the car i mean) and the single most important thing is to drive with a light right foot! a given car's fuel mileage can vary within 15 or more mpg between consecutive tanks only by changing driving habits!
good luck