Mechanic Q: Fuel Gauge?

Fuel gauge reads irratically, could show 3/4 tank on one start up then show 1/2 tank on another. Also gauge may fall then rise then settle down. I never know for sure how much fuel I have.

Mechanic Q: why does my truck downshift at the slightest grade?

While in cruise my truck will downshift at the slightest of grades but when I punch it for passing it doesnt.