1991 Chevrolet Truck USES WAY TOO MUCH FUEL

Does your friend have the same size engine? There's three, and you have the biggest one.

You've probably looked for signs of leaking already. If there's no black smoke from the tail pipe, you'll need a scanner to view the fuel trim numbers. That will tell you if the Engine Computer is adding or subtracting fuel from the factory-programmed values. If it is, you need to look for the cause of that. Look for vacuum leaks, exhaust leaks ahead of the oxygen sensor, leaking fuel pressure regulator, and a misfire.

If the fuel trim numbers are near "0", look for things like dragging brakes and tight universal joints.