I have a 50 gallon tank and normally put in 2 or 3 gallons more than it says I have used. I try to fill when gauge shows a quarter tank.

 

I do not watch the fuel gauge as much as the trip meter. I reset the trip meter each fuel stop and check the fuel quantity used. I know my 65 gallon tank uses 10 gallons before it comes off 100% fuel level on my CTS2. Once it starts to move, it is in concert with the factory gauge verses the CTS2.

 

When I had the dealer reprogram mine in Fort Worth the shop foreman came and asked me if I wanted to select 45gal or 55gal for the reprogram as 50 isn't a option on their software. Wanting to err on the side of too much fuel left vs not enough I chose the 45gal setting. The miles till empty might be off a bit but fuel used on the trip meter is always within .5gal used when refilling at the pump.

 

When I installed mine, I put 4 gallons in it then drove two miles to the station and filled it up. I almost always fill it at 1/4 left and almost always put around 40 gallons in it. The trip meter is higher by 1/2 to 1 gallon used. I don't like anyone putting fuel in my truck so sometimes I might fill it up at a half tank just so the wifey doesn't have to fuel it up if she needs the truck for something. This was a great upgrade to the truck. It is nice not to have to fill the truck up before every trip.