I am having the same issue with my 90 trooper. I fill up and it will go about 200 miles to empty. Who knows the size tank for a 90 trooper?

Tank size does not matter.

How many gallons did you put in per those "about" 200 miles.

Next time take the exact number of miles driven since the prior fill up and divide it by the number of gallons it takes to fill the tank. Tank size, "about" miles and "about" gallons are not relevant. You need to know the actual miles and gallons to get a real MPG figure.

Michael