What (in detail) can one do to their older (FORD, CHEVY, GMC) full sized car, truck or medium sized box truck, that only get 10 or less MPG on a good day, to be able to drive it long distances constantly or short distances in busy cities, while getting good MPG (20 mpg or more) without compromising standard performance or going broke trying to function. I know there are ways to get 50+ miles per gallon on the biggest of vehicles without destroying your car, having to by another one and or going broke either way. Please Genuine Tried and Tested Knowledgeable Mechanic Gurus Only. I need help!!!!!!!!!!!!!!!!!!!!!!!!