Gas Mileage droping

It could be a wide variety of things. Can you list everything done? It could be the distributor or coil packs starting to go. Or an oxygen sensor. If you start it up and stand behind the exhaust does it smell as if the car is running rich? If so it isn't burning the fuel properly. It could also be that you have a small fuel leak in a fuel hose. Inspect the fuel lines while your at it.

I had a complete tune up done, Spark plugs and wires replaced, all fluids flushed and filters replaced. There was a head gasket leak last year and half the engine was rebuilt including the timing and distributor cap. The transmission was rebuilt about a month ago. When I first start it every day it reves up to around 1700 rpm, gradually dropping to about 1000 rpm and then not going down to 700 rpm until I put it in gear. I had it check for a vacum leak and there are no codes that come up.

I had a much similar issue about a week ago. Apparently I broke some wires that are responsible for sensors during cold startup. My car would idle high and then drop to 1000 and when I put it in gear it was around 800. But it also would accelerate and stall on its own. I took it to the mechanic and he fixed it in under 10 minutes, wouldn't even let me pay him. Ur best bet is to just take it to a mechanic there's too many electrical parts to these cars.