my mom's Z has 205/55 r16's on it. Upgraded full exhaust system usually improves mileage by 1-2mpg on most vehicles if your driving habits don't change. I once drafted for the majority of a trip from Orlando to Tampa on I-4 and achieved 24mpg on my Z when it was non-turbo. You could also convert your rear end to the taller geared TT differential and that would bring your cruising rpm down significantly. Maybe do an electric radiator fan too. I would assume it's possible to achieve 30mpg tops if you do all those things but gas mileage is not what these cars were designed for. Here are some good websites (click on the underlined names): Z1 ConceptZ SpecialtyZ