Sounds to me a problem I have not seen since GM's of the 80's. Sounds like the fuel gauge is making dramatic adjustments to how the fuel is "sloshing" in the tank. My '84 Camaro did that all the time. When stopping fast or accelerating hard, I can have half a tank in reality, but the gauge read full or empty for a second or two depending on what's going on.

According to the shop book, the tank is 17 gallons. The low fuel light should activate when you have 2.2 gallons or less and go off when you have 4 gallons or more.

You said your dad replaced the pump. Prior to the pump being replaced, was the gauge fairly accurate? Also, when he replaced the pump, did he replace just the pump or did he drop in the "kit" that was the pump, float, sending unit, the works all in one??? If he replaced the sending unit with an aftermarket one, I am guessing that is the problem.
When you enter after market parts, there may be small things not quite the same as OEM. Such as, some guesses I have would be the float may not be the same length, so it will read out of fuel faster, GM may have done something with the OEM sending unit to reduce eliminate dramatic fuel read differences from fuel sloshing in the tank (such as delayed signal updates, so the fuel can settle before the gauge reacts).

 

its an 18 gallon tank. Because i have ran mine way low and held 17.6 gallons.

 

Just saying what the GM Shop Book (not Chiltons or Haynes) reports. Per GM, it's a 17 gallon tank.

BUT, I will bite that some how you can get more then 17 gallons in the car. Maybe it backs up in the fill tube, maybe GM deliberately did not publish the true capacity of the tank. Who knows....

 

Not doubting either of you But when I fill up from "E", I only get about 15 gal. and the needle is past "F" . Then I do my 70+ miles before it moves. I'll bet GM did something like what Jason said in order to prevent running out of gas! Who knows?

 

when i'm on E, it takes only 12 gallons. I just take my chances and pray that for some reason my gas gauge doesn't decide to be accurate sometime! LOL!

 

According to GM...
My Monte holds 16.6 gallons, But then again mine is a 1999.
My gauge does fluxuate.... brakeing/acceleration etc...
Its always been that way even after I put in a new fuel pump (factory replacement) from Oreilys... 2 yrs ago.