First of all, 9 MPG isn't that bad, but with a stick and overdrive you should be able to do a few MPG better, especially if you keep a steady speed and don't tax the engine.
You're going to need to play detective here, but that also means getting to use some fun diagnostic tools!
My first step would be to do a major tune-up on the engine if you haven't done one recently. The major tune-up includes almost all of the investigative work you'd do for a gas mileage complaint, and it has the added benefit of turning known unknowns into known knowns, so to speak.
Take a look at the
Engine Diagnosis and Tune-Up Training Manual for further detail.
It's impossible to isolate a cause without doing some investigation, and with a new (to you) car, it could well be multiple small things adding up. Some of these would also result in other symptoms, so that could help pinpoint the issue.
Some causes:
-Faulty carburetion
-Incorrect initial ignition timing
-Low compression
-Faulty distributor mechanical advance
-Restricted air cleaner
-Faulty/leaking vacuum advance
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