We are planning on hiring a car but we dont know exactly how long we want it for.... somewhere around the 3-4 week mark Im thinking. Has anyone ever hired a car and had a flexible 'return to base' date? Or am I delusional?

I know that if you reserve for 1 week (which is usually 5-7 days) and return at 4 days you could be in for a nasty surprise - some companies will charge you the rack daily rate and you won't get your cheap weekly rate you think you booked.

Since you're renting for multiple weeks I see less risk of surprise. Best bet is to call up a few co's directly and see what they say.

Returning early usually substantially increases your rate. Returning late can incur huge fees/charges.

I once had to rent a car "open ended" during an insurance claim. That time it had to be done in weekly increments - I had to extend another week by going in person to re-sign the forms. So, you should contact the individual rental companies and get their advice/suggestions and if they have any provisions for 'squishy' rental periods.

I've had a lower price than the quoted when I returned earlier than I expected, once with alamo and once with thrifty. In both cases it was for a rental of more than a week and I returned a day early and was charged a day less, 8 days (a week plus 1) instead of 9 (+2). If it had been less than a week, I'm not sure what would have happened.

Typically if you rent at a long-term discounted rate and return early you will be charged a higher rate for the time you do use (as in the example of renting for a week and returning in 5 days). But if you rent for a lengthy period and extend the rental while you have the car you might be charged the more expensive daily rate for the extra days - but it shouldn;t affect the lower long-term rate for the time originally rented.

And yes, I have extended a rental - you can typically just do it by phone - but that was adding another day to a 4 day rental - and I was just charged one more daily rate. Just be sure the credit card you use originally has enough credit available to cover the additional charges.

Unfortunately, you're delusional. The contract you MUST sign when you rent a car will specify the EXACT date you intend to return the car. It also specifies what you will pay if you violate the contract -- usually substantially higher charges.

I should note that most contracts specify that, if you are two days past the return date without contacting the company, you will be considered as having stolen the car, and the police WILL be notified. You do NOT want to be arrested for grand theft auto!

You CAN re-negotiate the contract at a later time (ie, ask for a longer or shorter time period), but you're at the mercy of the car rental company. Remember -- it's THEIR property and they have YOUR signature on a contract. You won't have a lot of negotiating strength at that point, other than seeking their good will.

I can't imagine any company that will allow you to rent a car for which you don't specify an exact day of return at the start of the contract.

How would the car rental company be able to have a car ready for YOU if the people who rented that car before you just planned on returning the car "maybe this week, maybe next week"?

There's a simple solution to your problem, though you would not be able to lock in a rate ahead of time. Rent a car for 1 week or 2 weeks. Then as the time progresses, look for a new rental online for the next week if you decide you still need a car. You return the car and pick up a new one (or simply keep the one you have if you like it and are renting from the same company). Eventually, you will have to commit to the timing of the last rental period, and this is certainly not a way to control costs. But it is a way to keep your plans a bit more flexible. However, if you are planning to keep moving from location to location around the country, then this plan will not work because you'd incur huge drop-off fees.

However, some car rentals can be extended by simply calling the company and asking for a later return time. This happens all the time, but it's easier to arrange from a company like Enterprise that doesn't specialize in airport rentals.

My experience has been if you rent for more than a week, then additional days (over the weekly increments) are prorated on a daily basis but you must keep the car at least 7 days. I would say reserve for the 4 weeks you know you want. I think that is the maximum rental allowed before having to sign new docs anyway. If you need it for 3 weeks and 2 days, ask what the charges would be just so you know.
My info may be outdated so verify with the rental company. Things are always changing.

Regarding the question "How would the car rental company be able to have a car ready...?"

The answer is quite simple - a well run company figures out how to plan for these contingencies.

They have years of data they can mine for trends - how many cars are returned on time vs early or late, and make predictions. Same way the airlines plan for a certain % of no-shows or a factory must plan for increased demand.

They will keep on hand a certain # of cars for walk-up rentals and other uncertainties, and factor these costs into their bottom line and top line projections.

They will have good computer systems that alert them to potential problems (i.e. inventory falling low) and they'll have plans to shift inventory around if needed.

A poorly run company does not plan as well.

Also regarding the same question, please dont think I am an idiot who thinks I can waltz around the countryside and return cars when and where I feel like it!! I was just asking here as I know many Fodorites likes having flexible itineraries and I was wondering how you incorporate car hire into that. Thanks for the other helpful suggestions I'll call around and see what the companies say.

Another thing to keep in mind is that most Visa and MC cards will only cover your collision insurance for up to 15 day rentals, and they will not cover "back to back" rentals. So, if you're counting on that type of coverage...you'll have to rent twice with two different cards- or use an Amex card (which covers up to 28 days, I think).

"many Fodorites likes having flexible itineraries and I was wondering how you incorporate car hire into that"

yes - many of us do flexible itineraries --but generaly they are of a fixed duration. (Especially when using someone else's car)

It is the bits in the middle that are flexible - usually not the start/finish.

Someone needing a vehicle for "maybe 3 weeks, maybe 4 weeks" would probably do best to commit to a rental of either 2 or 3 weeks, and then have another RESERVATION for the remainder of that period, should the need arise.

As most car rentals do not require financial commitment up front, it costs nothing to have a reservation in the system for a car you won't use.

If this is for a long, circular driving trip/vacation, one would plan her circle so that it gets somewhat near to the point of origin at the aforementioned 2 or 3-week mark, so that the car could be returned then, and/or the next reservation be picked-up then before resuming the circular trip.