Depends what year it was built. The standard tank was designed for 75 gal, and that's all the gauge shows.
Then the extender was fitted (it will be a Supplement to the manual, fitted after construction) and the gauge will often still only show 75 gal - wait till you have flown for 50 mins or so and it starts to register.
Or, get a new gauge which shows fuel to 91 gal.
Machines built after about 1998 have the filler cap moved up the fuselage a bit, so the tank will hold the lot without the exterior add-on, and the gauge goes to 91 gal.