I did a one week rental at my usual HLE location and used the free tank of gas PC coupon #113120. The way it is done is they deduct the price of the gas off the base rate, then it is subtotaled. The CDP discount is taken off, then the gas price is added back on. Then the taxes, etc are added to that. In my case, my HLE took care of me & took off the extra amount as an adjustment to make it to my original amount. My HLE gets a 10/10 for service! I can even leave my personal car in their parking lot if I need to. It is only 2.7 miles from our house so we usually walk between home & HLE for some good exercise.
If you are at a high tax location such as Phoenix, Austin, DFW, etc, the effect is worse.
Bottom line: Bring out your printouts when you turn in the car so you can show them the price you're quoted.