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1. Travel by train or bus instead if you have the option.
2. Purchase carbon offsets for your trip.
3. Make sure your tires are properly inflated.
4. Drive the speed limit - driving faster uses more gas.
5. Pack organic, vegetarian munchies for the road.
6. Go to rest stops instead of getting off the highway, sometimes the gas stations and food places are much further from the exit than expected.
7. Use the AC sparingly. Air conditioning uses gas as well.
8. Don't idle.
9. If you have the option, drive an electric or hybrid vehicle.