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Something that will probably help with you gas mileage is to stop using premium high octane fuel.
High octane fuel is designed to be harder to ignite. This is to reduce the tenancy for fuel to prematurely ignite in high compression engines. Model A's even with the higher compression heads are still a low compression engine. Put the lowest octane fuel you can into your car. It will give you more power and most likely better mileage.

In regards to the GAV as I understand it, it is not a good idea to close it all the way as it can cause the engine to run too lean and possibly cause engine damage.