very bad gas mileage

[HACKGT350]

I'm driving my moms 00 jimmy while i am looking for a new truck (recently in an accident). im getting about 190 200 miles to a tank and all i do is highway driving. the numbers come out to about 16mpg and i know that isnt right. in the spring i replaced the plugs, wires, cap, rotor. a year ago i replaced the coil and shortly before that i replaced the MAF. the truck has about 160k miles and runs great and still has a lot of pep.

 

[DrPablo]

Don´t you have a 20gallon tank? 200miles/20gallon= 10mpg
I get also between 16 and 18mpg. I have a GMC Jimmy 1995 with 4.3l V6. What for a gas are you using?
At my other car I used always regular and when I changed to premium the mpg increased 2-3mpg up.
At fueleconomy.com you can compare different cars, and the Jimmy is located at 14-18mpg. Your gas mileage sounds about right.
You can also check your tire pressure. If it is too low your mpg goes up.