Friends of ours always rent a car when they take long driving trips. The reason: they say the cost of the rental gas is far less than the cost would be to drive their own car for the trip (assuming something like 50 cents a mile or so I'd guess.)
These people are pretty frugal and quite smart, so I'm guessing they've done the math, but I'm not so sure. My bet is that there are instances when it's a good deal (when you can get a discount, rent from a cheaper market, return the car to the place you rented it from (versus a one-way trip), when you need a bigger/smaller car than you have, etc.) and instance when it's not (you are only going one way by car, you don't need a type of car different than yours, your renting market is expensive, and so on.)
Anyway, I thought I'd throw this issue out for discussion. Has anyone considered the idea of renting a car for a long trip versus driving your own? If you have done it, what were the advantages/disadvantages? If not, why now?