i have a 2003 Cadillac CTS that i just bought used, im worried because when i drive it around, it says I'm only getting 11.2-12.8 mpg, and the little gauge is going to 75% full to 100% then to 80%, it just keeps jumping around, any idea why and or how much this will cost to fix? the people at Valvoline recommended a glass pipe cleanup or something like that... if i do that, will it be "right" again?