Ok but I get how to calculate it but I still only get 150 to 200 miles on the trip meter before I fill up I get about 2 weeks per tank

 

How low are you going for "a tank"?

Top your tank off. Use the slowest fill setting, and the second it turns off, pull the nozzle out. Drive for 100-150 miles. Top your tank off. Use the slowest fill setting. How many gallons did it take? How far did you go?

 

well I have a 98 2wd 5 speed i put on 31" tires and if I drive 52-54 I can get 24mpg just don't want to be in a hurry

 

So yesterday I filled up with 16 gal and went about 226 miles before I had to fill up it was all highway travel I got about 14 mpg from that

 

That's not terrible, but I would expect a little better. When was the last time you replaced the spark plugs/ignition tune up? Does your exhaust smell extra gassy? What is your tire pressure? What size tires?

Another thing, how cold has South Dakota been recently? That can hurt your mileage a lot.

 

Here is ny 2 centss.

I've got a 3.9L 1998 Dak and, even with my HEAVY FOOT, i can get 535km per tank (335miles) approx. Don't forget that in here in Canada now a days, the average temp outside is -17C in the morning, cold enough to make the truck run in DIRECT, No lock-up of the torque converter until may be 35 miles ....soooo sad.
You see the dollars running out the tail pipe....(laugh)

My worst experience was a month ago, in a snow storm, i had to travel 200km (120 miles) and it swallowed ALL the fuel tank , it was full when i left anf almost empty when i arrived in Montréal, Leaving Ottawa....running 75% of the time in 4X4 mode...

Conclusion, yess there is something wrong in your fuel concumption my dear sir.