1999 Chevy Malibu BREAKS AND FUEL

Your issues seem to be separate, though it is possible for one to affect the other.

First, your car has over 200K miles on it, and you can expect the engine to be a gas guzzler at this age. You can perform an extensive tune-up to breath a new life into this motor, but I really don't think that would be cost-effective. If you were to call around to a few junk yards, I'm sure you could find a used motor with under 60K miles for about $400, and a mechanic who'll install it for another $500. This is surely going to be more cost effective than a major tune up on a motor that's ready to die anyway.

But if you insist on saving your baby, before you attempt a tune-up, have your engine's cylinders "Compression tested". This will give you a general condition on the internal wear of your engine and determine if it's worth saving or ready for the junk yard.

An extensive tune-up would consist of:
Cleaning out your fuel tank, installing a new pump, pick-up strainer and fuel filter. Then test your fuel pressure regulator and replace it if it's performing out of spec.

Then have your fuel injectors flow tested. If any of them are faulty, replace them all. DO NOT have them cleaned or settle for some fuel system cleaner in a bottle. That is a waste of money (another story that I won't get into here). If they flow within 90% of spec, don't worry about them.

Next have your ignition coils and module tested. If any of the coils are out of spec, replace the entire coil-pack assembly. then replace your spark plugs and wires. (Make sure you use the factory plugs and that they are gapped properly)

Next replace your air filter, PCV valve and have your EVAP canister and it's components tested. If any fail, replace them.

Finally, have all of your engines computer controlled sensors tested. If any are out of spec, replace them.

Now as for your brakes:
There could be several possible causes to why they would grind under certain conditions and why they would wear rapidly. But what comes to mind first is a faulty brake caliper/piston/seal.

This would cause your pads to stay in constant contact with your rotors, which would in turn cause the pads to wear rapidly and would also create a high rolling resistance which would give your terrible gas mileage. You should take your car to a brake specialist and describe your symptoms and ask them to check your calipers.

And don't forget, you could also have worn wheel bearings that can make this grinding noise as well. And it will often sound like the brakes.

Good luck.