Make sure your car is running in optimal condition. Clean filters, etc, and Tires are properly inflated. Don't drive fast or slow down too quickly. G35's aren't known for great mileage. Are you using premium fuel, if not you will get less mileage. How far off are you to the estimates and what mileage are you getting now?
I have a MT, I get ~22.8 mpg on 89 octane and ~23.5 mpg with premium. Anything from 19 to 26 mpg is normal in the MT. If you consider this terrible you could buy a Prius. Why stop there? Get a Nissan Leaf, you can build trees!
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