My 99 seems hard to fill up, the pump shuts off early and I can usally get a few more gallons in it if I spend the time, but even when I think its completely full my gauge never reads full so I guess mine is the opposite of others.
Gauge issue for sure. I have a 1/4 reading when in the 20-24 gal fill up. when trucks first came out someone tested when light comes on 6 gal left. I've tested farther than that by always reseting the trip meter but always use that as my base. When gas pump auto shut off can always depending on pump add an easy 5 gal more by running it slow. I've run low whole bunch of times. Tanks are rated at a certain temp and weight. The worst or the best depending how you look at it i put in 32 gal filling right up to the neck by drizzling it in