Everyone knows that airfares can change unpredictably, and it's largely a guessing game as to the best time to book. Car rentals prices can also fluctuate--wildly at times, and there's no way to tell you're getting a deal except to continue shopping around after you've made a reservation.
A few weeks ago, I needed to reserve a car for an upcoming trip. I had done searches on Orbitz and Sidestep, which shop multiple agencies at once, and wound up booking a mini-van with Thrifty that then had the best price at the time: $281 per week, plus $48 per extra day. With taxes and surcharges, my itinerary was estimated to cost $406. I had used the company in the past and was a member of its rewards program, so felt comfortable with the reservation.
But because I knew that rental agency rates change from time to time, I priced out the same itinerary more recently. This time, the rate quoted--for the same dates, same style of car--was $199 per week, $34 per day extra, bringing my projected total, with taxes, to $287. That's a $119 difference! Unlike with flights, there's no charge for changing or canceling car rentals, so I quickly dropped the original reservation and made a new one.
The moral is: If you looking for the best deal, no matter which car rental agency it's at, your job isn't done simply because you've made a reservation. It took all of two minutes for me to save over $100--and you can bet I'll continue to periodically price out the rental until the day the trip begins.