The next time you need to book a rental car, try the Half-Hour Hack.

Want to lower your price in an instant the next time you book a rental car? After you get a quote with a pickup/return time on the hour—for example, 9 AM or 1 PM— get a second quote for the same car and dates but with the pick-up/return time a half hour before or after your original time—for example, 9:30 AM or 12:30 AM. You may be surprised at how much you save.

Skeptical? Check out these examples. Recently, a week-long midsize SUV rental at Denver International Airport (DEN), pick-up and return at noon, came in as follows: Thrifty, $75/day; Dollar, $77/day; Alamo, $95/day; Budget, $108/day; and Hertz, $137/day.

A quote request for the same car on the same dates at the same airport, but with the pick-up and return times put back to 11:30 AM, came back like this: Thrifty dropped to $64/day, Dollar went down to $66/day, Hertz went down a little to $135/day, while Alamo and Budget stayed the same. So if you booked with Thrifty or Dollar, you’d save over 14% ($77) just by adjusting the pick-up/return time by thirty minutes.

For a recent quote at Phoenix Sky Harbor Airport (PHX), the price on a five-day rental of an intermediate-sized sedan from National dropped $152 by shifting the pickup and return time by 30 minutes.

Why does the Half-Hour Hack work? The price drops are not coincidence. Often car rental brands price their rentals lower on the half hour than on the hour. Why? Think pricing algorithms, automated rate shopping, and a desire to catch competitors unaware in order to swing more business their direction.

In other words, the rental companies are re-shopping each other’s rates constantly and adjusting their pricing to try to get a leg up on the competition. These companies know some percentage of customers will choose to pick up and/or return on the half-hour; in those cases, by offering lower prices, they will win more of the business.

Think of car rental pricing like a water spigot. If you are the lowest-priced option in the market, then the spigot is on and reservations are flowing. As soon as one of your competitors undercuts your pricing, even by a small amount, the spigot turns off and the reservations stop flowing. A lot of customers will always choose the lowest-priced option, so it’s an effective strategy.

Twist’s Take: The next time you book a rental car, get two quotes—one for pick-up on the hour; the other, on the half hour. You may save significantly with the latter!