Several fuel centers have loyalty programs, and Kroger's Shell program has some problems, but is working well for me. One problem I've heard about is that your Kroger card doesn't scan at the pump at Shell stations making it inconvenient for people who prefer to pay at the pump.
Here's how it's working for me:
For every 100 points you earn, you receive 10 cents off per gallon of gas you purchase at one fill-up--up to 35 gallons.
You earn points by purchasing groceries at Kroger, 100 points for 100 dollars spent, and by purchasing gift cards at Kroger, 100 points for every 50 dollars spent. You must by them in $50 increments to earn double points. In other words, if you purchase a $25 card, you'll get 25 points, not fifty.
Additionally, you can earn 100 fuel points for every two non-government funded prescriptions you fill. (But, since I usually see a line at their pharmacy, I haven't ventured into this area.)
I live close to a fuel center and do my grocery shopping primarily at Kroger, so I usually earn between 200-300 points/month that way.
But, now that you can earn double points on gift cards, I simply buy gift cards to other places I shop when I'm at Kroger. For instance, Target and Home Depot are common places I frequent, and Kroger carries sooooo many more gift cards: Amazon.com, Barnes & Noble, McDonalds, PF Changs, JoAnn's Fabric, you name it! If you often eat at restaurants, you'll definitely want to stop in at Kroger to check out the cards available. Be sure you are looking at their whole selection, however. They have small displays of gift cards in several places but usually there's an isle that has their full selection.
So, twice this month I saved $0.40/gallon and filled up with 19 gallons or more. A savings of more than $7.60 each fill up.
This program is definitely not for everyone, particularly if you tend to lose gift cards or if you aren't able to get a full tank at a Kroger or Shell station b/c you don't like to let your tank get low, etc. Also, the smaller your gas tank, the less savings you'll be able to get.
When gas prices skyrocketed one year, my husband and I pulled our cars in close together and filled up both our tanks on one receipt--since the deal is only for one fill up. So, if you have a big tank or can manage fueling up two of your cars at once, you'll get even more savings.
For a full description of the program, visit Kroger's page.