Newbie Questions for a '92 manual

[Moonwalker]

I picked up a '92 manual Civic yesterday. Needed a new car to get to work and in this market I knew I needed to act fast, so I picked this thing up without too much scrutiny. It started and drove, so I paid my money and got down the road.

The kid who owned it drove it back and forth to college but just joined the military out of state and didn't need it any more. He did some performance modifications, as well as some recent maintenance (master cylinder, timing belt). He also apparently put performance intake and a straight pipe. The dad (who was selling it) pointed out that the check engine light is on, and said something to the effect of "that's because of one of his modifications."

I'm not super experienced at diving manual so it took a little getting used to. But I was alarmed this morning driving to work when I saw my gas gauge start to drop fast. I'm getting about 70 miles to half a tank. Cabin smells like gas when car is moving slow, and engine is revving very high when in neutral.

I'm thinking I will take it to a shop next week and get a rental car... but in the meantime does anyone have any ideas? I am maybe not the smoothest shifter out there but this was mainly Highway driving.