Is it the gas gauge or sending unit/

 

1-Fill the tank
2-Reset trip odometer to 0.0
3-Drive a couple hundred miles
Then repeat above steps.

 

Flip a coin: Heads = sending unit; Tails = gauge.

The floater in the tank might be stuck. When you fill it up do you fill it 100% full? Driving with like 1/4 - 3/4 all the time never getting the float to the top can allow corrosion and then sticky situations.

Kind of a Chrysler thing, never heard of a gas gauge problem with an H3, but I am sure there must have been some as old as these trucks are getting.

 

Or you could use a Scan Gauge and follow its procedures for tracking fuel consumption. In addition to reading and clearing codes, and monitoring real-time OBD info, it will let you see your current mpg, short term average mpg, the current tank mpg, how much fuel you've used since your last fill up, how much fuel is remaining, and the estimated range on the remaining fuel based on current mpg.

The only downside is whenever you add gas you have to fill the tank all the way. It won't work on partial fills.