I know on the Optima hybrid that was rated for 17 gallons, folks have been able to get 22 gallons in it. I've never driven it down to get that but I have gotten 21+ on an attempt. Current car is rated for 15 gallons and I've gotten 20 gallons in, takes a while but it takes it. Got over 900 miles out of that tank. Can't say if that is true for the non-hybrids though.