You may need one more tank to be sure this isn’t an issue of the pump shutting off at a different time. However, let’s assume you’re correct.
Is your tire pressure low? After a long drive, is one wheel much hotter than the others (in which case that brake is sticking on)? Do you have a check-engine light on? Are you up-to-date on all of your maintenance? If this is an automatic, is your car properly shifting into the highest gear, with the torque converter locking up? Are you using the proper grade of oil?