When you get rid of the cost of motels and you tone down the expenses on food, the biggest cost of a road trip is suddenly pulled in to gas costs. While there's not a lot you can do to significantly lower your gas usage without investing in a hybrid vehicle, there's a few minor things you can do that can save you a ton of money while on the road.
Don't Go Too Fast!
If you go over 65 mph, your car starts to lose its gas efficiency by a pretty high amount. Be a grandma and drive slow. You'll get more bang for your traveling buck and you may just survive the ride.
Drive Behind Trucks
SOOOO not safe but if you want to save some gas then you may want to try coasting behind a large truck on the highway. It'll keep you from violating the tip above and it will significantly cut down on your air resistance which is often a big gasoline sucker. I've done this more times than I'd like to admit but it definitely has saved some significant gas when I did.
Neutral Is Your Friend
When you're careening downhill at breakneck speeds, don't forget to put the car in neutral to take full advantage of all that potential energy you've accumulated driving up it. Just keep an eye out for police cars that are waiting for your speedy butt to turn the corner!
Maintenance Is Key
Before you go on your trip, check everything. Make sure you have air in your tires and that all liquids in your car are at their full amount and your oil filter doesn't need to be changed. Don't completely rely on oil change companies, either. Feel free to take your car there if you don't know the first thing about cars but they don't always do the job they claim they will. Jiffylube promised to fill up my coolant after they replaced my oil filter and I found out later that they just drained it out and left it at that.
Leave Some Room
When you're filling up, don't fill it to the top because gas will evaporate on hotter days when it's filled to the brim. Although it may not be much, it can add up over time.
Open Those Windows
If you're driving at slow speeds (less than 45 mph), then keep your windows open in order to keep the car cool on a hot day to save on gas. If you drive faster than that then air resistance will increase and your efficiency will decline so close those windows when you reach the highway.
Use a cheap gas locator like gasbuddy.com in order to find the cheapest gas on your route. It can save you a good deal of money in the long run.