Unfortunately, there's no quick and simple answer.
The % discount with car rental promo codes will usually vary from company to company and may vary even within the same company depending on the dates and times, length of rental, car type and what rates are normally offered. You also need to verify that the car rental company offers a discount for the membership or programs you've mentioned.
See our Discount Codes Tips
You may find a higher % discount using a car rental promo code when pricing a midsize car rental than when pricing a minivan rental. If the car rental company is already offering a fairly low rate you may only get a 5% discount... if their rates are really high you may find that the rental car code gives a 10-15% discount... or more.
If the rental provider is already offering a special... say a $9.99 weekend day rate (like Enterprise offers currently)... you will probably find that a car rental promo code from one of those programs that you mentioned will not give a discount rate at all. Sometimes the car rental company will not go lower than a certain rate.
I'm sorry that this is probably not the answer you were hoping for.. I wish I could give you a specific number, but discount car rental percentages really do vary... you will usually see terms on discount car rental offers that say things like "up to 25% off"... emphasis on the "up to" part! Most discounts fall between 5 - 15 % off -- there are not many that offer 16- 25% off.
I'd advise to check several companies with each discount car rental promo code to make sure that you're getting the most value. If you have the promo code, you should enter it when reserving online, or ask for it if reserving over the phone. You can certainly ask for it at the counter, but you may find that this slows you down a lot at the rental counter.. because you've booked at a certain rate and they may have pre-prepared some of the paperwork in advance of pickup.
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Free Coupon Codes