Heres the dilio, I look at the fuel guage and its at less then 1/4 tank left, the DTE is listed at 50 miles. Im getting 22.4 mpg. So Logically, it seems to have about 2 gallons left.

I take it to the gas station for fill and it fills up @ 16.1 gallons.

so if there is a 20 gallon fuel tank, either the DTE is wrong, the guage is wrong, or the gas pump is wrong. What gives? This has happened 3 time so far, im at yellow on the gas, and I fill it up and its 14, 15 or 16 gallons, Theoretically I should be able to go another 80-120 miles!

Anyone have any idea or similar experiences?

 

The DTE doesn't measure the mileage you've gotten in the past but more "right now". You can park the car with 20miles DTE, start it up and let it idle for 2 minutes; your mileage is zero and your miles to empty becomes zero even though you may have a gallon left. This makes for some white knuckle driving.
Also the DTE is according to "measurable" gas left - there's probably a gallon or so left after that.
How much gas does the tank hold anyway?

 

The tank is 20 gallons.

I think 16-17 gallons to empty (on gauge)...with a 3-4 gallon reserve.

 

I believe miles to empty is based on an 18 gallon tank (2 gallons in reserve). Based on your post, I'd say your MTE is working the way it is supposed to.l