Poor Gas Mileage

I actually owned this same truck. My wife drove it and I was a fanatic over the mileage. It is the most diverse mileage ive seen. Local driving would be as low as 13mpg to 17mpg. But on a trip 21mpg. To the other mechanics on the site this engine is suppose to have 93 octane fuel. Its mileage was horrible and my wife didnt like paying for 93 so she put regular in it. The mileage increased and I checked on every fill up. The only thing was not to do, heavy acceleration this was one of the big factors in losing fuel mileage!