Getting a Realistic Handle on What It Will Cost
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I've never run into the Mississippi as a determining factor in any of my rental car contracts, and I rent tow or three times a year all over the country. What I suspect is happening is that this is a subtle way to get their 'underage' driver fee out of you without actually saying so. For two drivers, assuming that you and one of your companions split the driving, that fee for both of you plus the 'additional' driver fee would typically add $75/day to the cost of the rental. I suspect that would be prohibitive in your case. In any event, car rental rates have risen significantly in the last several months, and I don't see you being able to rent any car for over three weeks for less than almost $1000, and that's before adding additional fees and/or mileage charges. You'll need to get your finances straight, including the real cost of the car rental, before you proceed too much further with your planning.
Another place where I don't think you're going to be able to hold down expenses as much as you seem to be counting on is in your sleeping arrangements. While you will fall asleep from time to time in the car, you will not get a 'good night's sleep' no matter how you try. You need to plan on being out of the car in a motel or at a campsite where you can stretch out and sleep. And just as a point of safety, the driver and the front seat passenger are never to sleep. The reason the driver can't should be self-evident. The reason the front seat passenger can't is that it is his responsibility to see that the driver doesn't. Also (not trying to be a downer here) I estimate your gas costs to be at least 15-20% higher than you're planning on. You have to factor in several things such as extra mileage getting to places that aren't on your direct point-to-point map, the fact that you won't be getting great mileage as you cross the Rockies, the higher cost of gas in outlying areas and at all the tourist locations you'll be visiting, etc.
AZBuck