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When do you get the best price for a rental car?

When is the best time to reserve a rental car? I'm planning a road trip in Arizona in the spring and wondered whether I'd get a better rate if I locked in a price well in advance, or if I'm better off waiting until the last minute. Do rental car prices fluctuate as much as airplane tickets?

I called some experts but didn't get a definitive answer. Rich Broome, spokesman for Hertz, said rates fluctuate "right up to the last minute." Others said car rental rates fluctuate very little.

Instead, I got advice: Reserve a car well in advance if I find a rate I like -- most agencies don't make customers pay in advance when they reserve a car. Then as my trip nears, check last-minute deals. If I find a better rate, I should take it and cancel my earlier reservation.

The downside, said Mark Kahler, About.com's budget travel guide, is that if I want a big car or a specialized vehicle like a SUV and wait until the last minute to reserve, the agency may be out of that particular vehicle. That's particularly true this time of year, when a lot of people travel and rental agencies run out of cars – or charge premium rates,

I often rent through Priceline.com. There's no trading for a better deal once my bid has been accepted, so I usually make my reservations no more than a week before I travel.

"If you’re a bit of a gambler," said Brian Ek, Priceline.com spokesman, "then wait until the last minute and try Name Your Own Price. When rental car companies see that they’re likely to have vehicles sitting on the lot un-rented, that’s when they’ll make them available through Name Your Own Price at discounts of up to 40 percent over published rates."

One online agency, Autoslash.com, says it will regularly check rates on behalf of customers who have booked rental cars in advance with them and get them the lower rate if the price drops. I haven’t tried Autoslash.com, but the Los Angeles Times said it regularly gets lower rates for its customers who reserve a car well ahead of time.

Now I'm intrigued. How much do rental car prices change? I've looked up rates through several agencies for my Arizona trip and will check periodically on whether they've changed. I'll let you know what I find.