Rental car companies, depending on the company, do have things that allow you to drive several thousand miles, but you have to pay extra. You will also most likely have to have someone else rent it for you, and you better not screw it up or that person is going to be mad.
When I went through the Rockies I went during September. I was on 80 but I cut down to 76 and then caught 70 in Denver, and it went relatively smoothly, especially considering the brakes on that car didn’t work too well. I definitely suggest acquiring a car with the low gear settings if it’s an automatic (my mom’s old Dodge Caravan that I learned to drive on DIDN’T have that, so I’m not sure if it’s a standard thing for automatics nowadays), or a stick shift, which is better for driving in the snow.
I’ve made several long road trips in my short time. This is my little bits of knowledge.
Get an atlas. They sell them at Wal-Mart for about seven bucks apiece, and the Wal-Mart ones also have a list of EVERY WAL-MART IN THE COUNTRY. This comes in handy, often.
Bring a pillow. Each of you. You’ll want one. Trust me.
I’m assuming you’ll be able to find a place to shower every night. If you think you might be getting grungy at any point, invest in a cheap box of baby wipes when you pick up your atlas. In fact, they’re good to have around for other things.
Also, keep a small supply of toilet paper in the car. Searching for a restroom in the middle of nowhere isn’t fun when you can just pull over by the woods somewhere.
The obvious suggestion is to get a cooler for drinks and some chilled snacks. The less obvious suggestion is to keep a supply of things like twinkies and energy bars and such in a console for the driver. Also, if the driver smokes, keep their cigarettes and lighter in easy reach.
My dad made me keep an emergency kit in my car which I still have to this day. Small rachet set, screwdrivers (both kinds), jumper cables, chains, wire cutters (for the chains, I think - never had to use them), flashlight, flares, a blanket, canned food and a can opener, forks and spoons, saltines, rags, a couple quarts of oil, brake fluid, transmission fluid, paper towels, a cheap prepaid long distance card, and pretty much anything else you might need in an emergency. Since six of you are going, I also suggest making photocopies of all of your ID’s and keeping them in a ziploc or something in there, as well as a list of emergency phone numbers, in case the worst happens and you all become incapacitated, or next of kin need to be reached (it’s a horrible thought, but you do need to prepare for it - just in case).
If you rent a vehicle, make sure you know where all of the fluids go, and how to properly jump start a car, before you leave.
Plan the route ahead of time and let every person’s family know where you expect to be each night. It sounds stupid, since you are all probably adults, but check in with them, at least every night, if not several times a day - if something happens and you guys go missing, get kidnapped, or break down in a place with no cell signal (it’s happened to me, and it’s not fun), then people will know something is up.
Consider investing in an AAA membership. They give out free maps and tour guides, and they will save your ass if you get stranded. It’s $50 a year and it’s worth it. Plus, some hotels give you discounts with them.
Check your tires and brakes before leaving. I’m serious - if I’d done that I wouldn’t have had a tire blow out at 95 miles an hour in the middle of nowhere in Colorado.
Make sure, incidentally, that someone you’re traveling with knows how to change a tire.
Get lots of batteries. Walkmans, discmans, and portable games chew them up like you wouldn’t believe. If someone has a Costco membership, buy them there - you’ll need them.
Make sure you bring all of your charging equipment with you - for your cell phones or laptops or portable gaming systems. That should be item one on your list.
That’s all I’ve got now but it’s a lot. I’m sorry. I just think back to all of the times I’ve driven long distances (I drove from eastern Pennsylvania to western Nevada and that sucked) and remember all of the times I’d said “Shit, I wish I had _____.” or “Wow, thank God I brought _____.”
Good luck, and have a safe trip.