2001 Rodeo 4WD?

In order to know that 16mpg is bad, you need to look up what the epa's suggested mpg is. If you're under the range, where and how you drive has the most to do with mpg. If you're in the city idling all the time and perform multiple accelerations, that'll eat up gas. Sitting in traffic gives you 0mpg. Also, depending on the engine, if it's a 4cylinder, it's trying to hard to move a non-aerodynamic brick down the road. The type / brand of gas you use may have minimal effect. Just make sure it's not billy bob's gas corner.