My gauge stopped going all the way up also, and I have an 89'
so there might be an issue with the older gauges just not reading
accurate like they used to.
Carry a Gallon or so of gasoline until you either figure it out or run
out of gas, that will tell you something about what's going on.
I used to get about 200-230 miles per tank highway, till I lifted
and built up my trucklet, if you are stock that would be a start
to figure out how far you can go.
Wild