I'm hypermiling my wife's car and she doesn't even know it
Of course, my mantra is that the best mod to make is the driver, but I'm helping my wife get better mileage without her knowledge. She already drives like a hypermiler in regard to accelaration and driving style. But she scoffs at my attempts to hypermile (like removing the towing package and luggage rack), so I decided to go stealth.
I am solely responsible for the upkeep of the vehicles, so it's really my own discretion. So far, I've over-inflated the tires and taken out a bunch of extra weight. She keeps meticulous records, so I'm going to see if it's helping at all. I'm not using additives in her car like I am in my Tribute. She's got a 2005 TSX and it's in pristine condition.
Naaaa just a small squirt of GP7 in the filler when the tank gets low so when she fills it will mix in with the gas. That is one thing that definately helps as well as provides a little friction reduction in the upper cylinder area of the motor and better fuel combustion.