An alternate opinion on saving money on the trip
Hello Djac,
You've received some excellent advice on planning for your first XC (cross-country) RoadTrip. Allow me to add an alternate view of economizing:
I personally find slow travel speed on the Interstate highways to be worrisome at best and downright harrowing at worst. While it's absolutely true that 60-65 mph speeds are best for most vehicle's fuel efficiency, that speed range is slower than probably 90% of the traffic on an Interstate highway. With some 23,000 miles of mostly Interstate travel from the East Coast to points as far as Nevada and Idaho under my belt since July 2010, my observations are that even tractor-trailers are running 65-70 mph, and they're almost all "governed" where the motor carrier corporate management has installed devices limiting the vehicle's top speed. The smaller number of tractor-trailers which aren't governed will run 80+ mph downhill most anywhere. Automobiles and pickup trucks/SUVs will almost all be running 75 to 80+ mph. On most of my noted RoadTrip miles, I limited my speed to 72-74 mph to save on fuel costs and that put me comfortably above most tractor-trailers and behind most automobiles. Even then, the faster tractor-trailers and the autos would often pass me at a rate of 10 or more mph faster than me. To see faster traffic flying up behind you and changing lanes at the last moment to overtake you makes for a white-knuckle drive, in my humble opinion. To do so at 60-65 mph such that said overtaking traffic is running +20 mph more than one's own speed is an experience I would avoid to whatever extent possible.
It is said "speed kills", but that's not what traffic engineers say. They say "speed differentials kill", meaning it's actually safer, if a bit more expensive, to run a bit faster than one's vehicle's most fuel efficient speed in order to make the speed differential between your vehicle and that of overtaking vehicles a smaller figure.
And as to how to turn faster average speed into savings, just play with the numbers a bit: You may find that averaging 70-72 mph will cost you little in additional fuel costs over your planned RoadTrip mileage, and if you can avoid even one night in a motel by traveling a bit faster, chances are you'll recoup that modest additional fuel cost.
If you decide to incorporate limiting your speed to levels significantly slower than most traffic, please do yourself and the rest of the traveling public a huge favor: Keep to the farthest right lane at all times. In the rare instances you'll be overtaking slower vehicles, turn your cruise control off for a few moments, accelerate to such a speed that you can overtake the slower one quickly, and enter the passing lane (left lane) only at such a moment when faster traffic is well behind you, and get back into the right lane before decelerating down to your chosen travel speed. Doing so provides for not imposing your chosen method of travel on all others, and prevents the dangerous situation of higher speed traffic accumulating behind you while you would otherwise overtake very slowly.
Have a safe and enjoyable RoadTrip!
Foy