At least for Kroger, you get $0.03 baseline with no discount.
After you spend $100, you get one fill up with $0.10 discount up to 35 gallons. My vehicle in particular has 11 gallons. So my maximum discount is $1.10, up from base line $0.33 discount. $1.10 discount per $100 spent is barely worth it.
Even if you used all 35 gallons, that's $3.50 discount per $100 spent.
So if you have a massive vehicle that takes 35 gallons, you can save $35 off the fill up after you spend $1000 in the store. Could you have saved that $35 by shopping at a different grocery store?