I fill at around 3/4 (on the gauge) for a few reasons. First, it's an emergency vehicle and I never know when I might have to drive a couple hundred miles. Second, the fuel gauge on this truck has been "wonky" since it was new. GM records show the whole thing (gauge and sending units) were replaced twice before I got the truck. What happens is, if I fill both tanks the gauge goes to about a 1/4 past full. (And will sometimes try to go further, then suddenly drop to E and stay there until the next time the truck is started. This once left me stuck on the side of the road with a full rear tank and empty front tank, because it will not transfer fuel between tanks when it does that.) The front tank is supposed to be 19 gallons, and the rear 21, for a total of 40. If I fill at 3/4 I can usually put in 20 gallons, which leads me to believe 3/4 actually equals about 1/2 on this particular truck. I've had it as low as a hair above 1/4 (with a full 5 gallon can carried in one of the side boxes) without any issues (took 38 gallons to fill)
I wish I could remember how many gallons it took for its very first fill up when I bought it. I bought it used, and the gauge was in the red when I drove out of the lot with it. Went straight to a station and filled it, and it took more than $200 (diesel at that time and place being $5.25gal) but I can't quite remember the exact amount and how many gallons. I think it was 42 or 43 gallons, because I thought for the longest time that it had two 25 gallon tanks, but later research told me the tank sizes that were standard on that C&C model. Could the shop that added the body have changed the tanks to something bigger? Sure, but without running it dry there's no good way for me to know.