Another investigation here. "Gma" investigates, should you do it? This morning, frequent flyer miles. What to do with them? And what not to do with them? We have surprises, with very experienced... See More
Another investigation here. "Gma" investigates, should you do it? This morning, frequent flyer miles. What to do with them? And what not to do with them? We have surprises, with very experienced travelers, like ABC's gio Benitez. Tell us what you found out. Reporter: It can be mind-boggling. It's a tough decision. You might find yourself giving up and blowing your miles on anything. We brought in the ultimate expert. The airfare watchdog in this morning's "Gma" investigates. Right now, here in Boston -- in just a year, I traveled to all corners of the country. Covering 300 stories. And living out of my suitcase. You know what that means? Lots of frequent flyer miles. And, boy, was I due for a vacation. I wanted to make the most of my miles. Which sounds easy, right? Not so much. There are constant changes to flyer mile programs. Like delta's recent announcement to reward for for how much you spend instead of how many miles you fly. It's enough to make your head spin. I called in the big guns. George hobica, the founder of airfarewatchdog.com. Who has air flyer miles to a science. The biggest mistake is not getting full value for the miles. You can spend 25,000 miles for a $200 ticket or an $800 ticket. Cramped in economy, with visions of first class, rewards may be your rescue. 15,000 miles to go to business fare. Reporter: One click. It's a bargain. Reporter: What about putting the points toward a hotel or a rental car? It depends on the price of the hotel. If you can use a $500 a night hotel for 50,000 miles or less, it might be a good deal. Reporter: What about rental cars? Not a great idea. Pay cash. Reporter: All right. I'll drive. Not in the mood for traveling? How about online shopping? And what about merchandise? Never buy anything with your miles online. However, buy stuff online through the airline's shopping mall. Just Google the term, airline shopping mall. You get bonus miles when you shop through those malls. Reporter: The takeaway -- before you spend your miles, calculate the highest value. A long-distance trip, an upgrade or hotel or rental car. And put youroney towards online purchases, not your points. And here's a little secret. Some airlines let you register for special mileage programs using any credit card. And it gives you free miles for eating at certain restaurants or shopping online. You learn something? Yeah. Absolutely. I know what to spend it on. And right behind me, who do
This transcript has been automatically generated and may not be 100% accurate.