A 15-cents-per-gallon rebate is available for 60 days after customers register online. Speedpass customer appreciation days are back. To express appreciation to customers, ExxonMobil is repeating a favorite promotion of 15-cents-per-gallon rebate of gas purchased with a registered Speedpass account. Beginning today, customers may register their Speedpass accounts at Speedpass.com/save to earn discounts for a…