Almost every major airline has partnerships with the large car rental agencies such as Avis, Budget, Hertz and National that allow you to earn air miles and points on your rentals.
There is a standard earn rate for each rental, or per day. You simply enter your frequent flyer number on the rental car website at the time of booking to earn the air miles. Depending on the airline program the standard earn rate varies, for example it could be 500 air miles per rental, or 50 air miles per day of the rental period.
In addition to the standard earn rate, the car rental companies often team up with the airlines to offer bonus frequent flyer miles on rentals, as well as discounts of between 20-40%.
These discounts, bonuses and promotions change often though. Here is an awesome site 'Frequent Flyer Bonuses' that shows you all active air miles and points offers for all rental car companies. This will help you decide what rental car company to go with and / or which airline program you want to earn miles with.
You can usually earn 1000+ air miles for a rental car booking with one of these promotions. To book using a coupon code, you can go through the airline website or the rental car website. You may want to check the rates on both.
It is also important to note that often, the coupon codes offering bonus air miles are tied to a specific rate, so you may be paying more just to earn the bonus air miles. It is always worth checking against the lowest available rate without a coupon code.
Additionally, car rental companies charge a per day fee if you elect to collect air miles. These fees are generally less than $1 per day. You need to do the math to see if the miles you are earning, outweigh the fee you are being charged. Airline miles vary in value, depending on how you redeem them, but for an easy calculation you can value them at 1 cent each.
If you were earning 500 miles for a 3-day rental and the fee was $0.75 per day, the math would look like this: 500 miles = $5 (500 x 0.01) and the fee charged is $2.25 (3 x $0.75). In this instance, it is worth paying the fee to collect the miles.
Another way to earn air miles and points on your rental car spend is by going to the rental cars website through a Shopping Portal. Many of the rental car companies partner with airline and travel credit card shopping portals.
However, you will not earn any air miles / points via the shopping portal if you use a coupon code that is not on the shopping portal site (and generally there are no bonus mile coupon codes on the shopping portal sites). So essentially you can’t stack earning miles through the shopping portal and through an airline bonus mile promotion.
I recommend booking through a shopping portal only if there are no good bonus air mile promotions available.