This wastes fuel and is harder on vehicle's components. It also increases the likelihood of an accident. Maintain your vehicle. Routine items such as proper inflation of tires, lubrication of moving parts and proper function of the ignition and emission systems will help your vehicle achieve maximum fuel economy. If you've got something in your trunk or other storage areas that doesn't need to go with you, take it out.
Heavier vehicles use more fuel.
Your owner's manual should be able to help you with this one. If a higher grade of fuel is not required, use the lower, less expensive one. Don't race a cold engine. Start driving when you start the engine, just avoid rapid acceleration. The engine will warm up faster and the exhaust-emission system will function sooner.