Many rental companies allow you to collect frequent flier miles with a wide array of airline partners, often at the rate of about 50 airline miles per rental day. Hey, free miles are always a good thing, right? But watch out--those miles may not be free.

Most rental agencies are now charging surcharges if you choose to collect frequent flier miles with your rental. These fees, which often range from $0.75 to $1.50 per rental day, are imposed to "offset a portion of the cost of purchasing the miles."

When you factor in the fees, the math usually doesn't add up. At the rate of $1.50 to earn 50 miles, it would cost you $750 to earn enough miles for a domestic round-trip ticket--more than it would cost to just buy the ticket! There are less expensive ways to accrue frequent flier miles than paying for the privilege to the rental car companies. However, there are some tricks which can tilt the scales in your favor. Here's what AutoSlash recommends:

  • Look into the rental companies' own rewards programs. Avis, Hertz, National, Dollar, and Thrifty allow you to sign up for their loyalty programs for free and earn points or credits towards free rental days with no surcharges. If you rent often, these can represent a great value, since it's not hard to bank up enough credits for a free day or two and then use them on an extremely pricey rental in a high-cost rental market. If you only rent occasionally, though, the program might not represent the best value, since it might take a long time to earn enough for a free day.

  • Pay attention to any promotions going on. Triple mileage promos or other bonuses can make the cost per mile much smaller, making it worth paying the surcharge for the miles. On the other hand, adding a triple mileage promotion code usually means removing a much more lucrative promotion code that takes real money off of your rental, so watch out for that.

  • If you have an account with a non-U.S. airline, choose to bank your rental miles with them (assuming the rental agency partners with them). Rental agencies usually only charge fees on domestic U.S. airlines. Miles you collect on foreign carriers like British Airways, Lufthansa, or Japan Air Lines usually won't cost you a thing.

  • Crediting rentals to hotel programs like Hilton HHonors, Priority Club Rewards, etc. are another way to get around the surcharges. While each rental doesn't get you a lot of points, those points are surcharge-free, so there's little downside to adding your hotel rewards program onto your rental contract to get a bit closer to a free night or upgrade.

  • Or, bank your rental miles with American Airlines. As of now, most agencies don't charge the full 75+-cent-per-day surcharge on American miles. Instead, they just pass on the 7.5% excise tax they pay on the miles, which amounts to about six cents per day. Six cents is better than a buck or more! (Hertz also does this for Delta in addition to American.)

  • In the worst case, if you have miles about to expire and need some activity to restart the expiration clock, throw your frequent flier number in. It's better to spend a buck or two than lose several thousand miles.

Mileage earned and surcharge amounts vary by rental agency and airline, so be sure to check out your rental company's website to see exactly how much your miles will cost and how many miles you'll earn.

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