I also have a 2005 Chevy Duramax Diesel with the same issue. I bought it brand new and it has been that way every tank. I filled up last night when the warning chimes alerted me that the fuel level was low (less than 50 miles range). Because diesel foams terribly I am the anal guy that fills it to the rim slowly by letting the foam settle off and trickle the fuel in. Filled to the neck it held 19.4 gallons. Tank size is the same as the qx at 26 gallons. Fuel gauge was in the Red below 1/4 tank also. So I still had a little over 6 gallons of fuel left and it was warning me I had three at 19 mpg roughly.
So I agree with the other poster all manufacturers are going to add in a safety reserve for the ones of us that will push the limits of a tank. You guys know the excuses: "I'll fill up this afternoon after work", "Running late this afternoon to soccer practice, get it tomorrow am" "While warming the engine tomorrow am, I'll fill up this afternoon so I won't be late to work this am", etc etc etc.
Facts are that running engines out of fuel is a bad thing. Running engines low on fuel is a bad thing for the fuel pumps. I've been fortunate to never have to replace a fuel pump in a vehicle in my lifetime. I firmly believe part of that reason is because my brother and my Dad are both mechanics and have taught me to never run lower than 1/4 of a tank when possible because doing so makes the fuel pump work harder as the fuel sloshes around in the tank while moving and when really low in the tank not only does debris get pulled in along with water,etc, there are periods of time when air will also get pulled in creating loss of pressure, etc....
Long story Longer...Lol...My roommate from college was the person I described above. Always pushing the limits of a tank of gas. Never putting more than $5 or $10 in at a time (obviously that was way before today's fuel prices). I know of at least four times he has had to replace fuel pumps on vehicles he has driven. Coincidence? I think not.
Point is while it is frustrating for us as consumers and drivers to not have accurate fuel gauges the reality is that you are going to have to fill up eventually anyways right? Does it really matter if you fill up today vs tomorrow? At Mile marker 150 vs 250?
For me personally there are so many other things I enjoy about the qx I don't mind the fuel gauge issue.
Sorry for the rant. Wasn't intended to be one or aimed at anyone in particular just sharing experience and advice fwiw. Lol.