Thanks JB. End of year vacation to burn is helping progress lately.
Regarding the decision to rent a car for long trips... I mentioned gas mileage, but that is actually a very small piece of it. Its really comes down to wear and tear and depreciation on our own cars. This is a very rudimentary way to look at it because its not linear, but I took the purchase price of either of our cars (around $40k each) and considered their useful lives (200k miles). This works out to about $.20/mile. For a 2,000 mile trip that we just did, that works out to $400. Rental cost was $250, so we were already $150 ahead.
Gas money savings for a vehicle getting 36mpg vs 18mpg, and savings on maintenance items like tires and oil are just gravy on top. We also avoid having to decide which vehicle to take (we both hate putting 2,000 miles on our cars in one week). I'm normally one of those back row angle parker type of people, so having a rental car with the whole family in it, I don't get yelled at for parking in the back forty because I now can park in the closest spot I can find. Another cool thing is just driving a different and late model car for a week, and playing with its gadgets. My truck is 7 years old now, and tech has advanced quite a bit.
Your insurance coverage should be checked before renting a car because if you have to take the rental car insurance, the math goes out the window. Luckily mine covers me in a rental the same as in my own vehicle so I can decline the expensive insurance offered. Lastly, make sure you choose a rental company with unlimited mileage!!
I hope this helps. Your conditions will vary, of course.