There is an Amex promotion running until 9/30/16 regarding saving 10% on gasoline (up to $250 purchased, meaning max $25 savings total).
It is easy to sign up, so it's worth doing, even though $25 isn't a lot of money.
Just log into your Amex account, and scroll down. You will see a ton of offers. One of them at or near the top will be this gasoline offer:
Just click "Add to Card", and it's activated. It costs nothing. You will instantly get 10% back off all gasoline purchases on that card until either Sept 30 arrives or you've bought $250 in gas on it.
You may see other offers you also want to opt-in.
Also, if you have several Amex cards, you can do this multiple times.
I have 4 Amex cards. It worked on 3 of the 4.
I am already seeing the 10% back on my gas transactions. Ignore the language in the fine print claiming there will be a delay. There won't be.