Why has my fuel efficiency decreased sharply?

If you've noticed this from the time you had the timing belt replaced, it may have been installed improperly resulting in the valve timing being off. Or maybe a sensor or two or a vacuum hose didn't get hooked back up?

Aside from that, it literally could be anything with the information that you've given. Bad O2 sensors, failing catalytic converter, transmission issues, failing fuel pressure regulator, clogged fuel filter, failing ignition coil... I could go on and on...

There are a few visual checks you can make. Check the oil and the coolant and make sure that you aren't getting one in the other (would indicate a blown head gasket). Check the odor of the exhaust. You don't need to stick your nose in the tail pipe, just stand by the back of the car. If the exhaust smells like antifreeze, that means that you have a blown head gasket, warped head, cracked head or cracked block. If it smells like gasoline, then you most likely have some sort of sensor problem. On a dark night open the hood and look carefully for any electrical arcing that would indicate bad spark plug wires. You can also in a very safe situation get the car rolling and put it in neutral and make sure that it's rolling freely. I've seen cars drop mileage like that because someone put power steering fluid in the brake fluid reservoir and it caused the brakes to stick. You can take it to places like AutoZone and have them check for free any computer fault codes that may come up. This may catch something like bad O2 sensors or bad intake sensors (MAP/MAF/TPS/etc.).

Again, I could go on, but that should be enough to get you going...

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a/c efficiency does drop with an increase in outside temperature, for a couple of reasons; 1: there is less temperature differential between the outside air and the condenser, thus less heat is removed from the refrigerant. 2: the heat load when the temperature goes up, increases, sometimes substantially even though the temperature might go up only a few degrees. the engine load doesnt change when the a/c is on, regardless of where you have the temperature setting, however the compressor will run longer. as long as the a/c system is in good repair, and properly charged with refrigerant, that is when you get the best efficiency from your a/c.

For that kind of a decrease in mileage I would be looking at a fuel system leak or theft rather than an efficiency issue.

Without knowing the year and which engine it has, I can't say for sure whether the engine is an interference engine or not. It's possible that the mechanic didn't get the engine back in time. You will need to get the timing rechecked>

Take the car to the dealership. Cars today are extremely complicated. Hand the problem to an expert.

http://www.youtube.com/watch?v=8Q7FFjUpVLg