In my case, I thought I had a leaky injector due to my mileage dropping to 15-16 tops, during highway driving with a VERY gentle foot. When I first purchased the truck, I got 18-19 mpg in similar circumstances. When I took my truck to the dealer, they claimed I had clogged injectors, not leaky ones (they did perform a leak test, but according to them my injectors were fine - considering their admission that leaking injectors are a common problem, I took it as a willingness on their part to replace my injectors if needed). The dealer also strongly suggested that I "drive it HARDER!" Although I was skeptical of the dealer's claim, I paid for an injector cleaning and stringently monitored my fuel consumption. I also began adding Stanadyne to every tank. End result: after 2 months, my mileage is now back up, with my highest being 19. 3 under LIGHT highway use. Towing (7500 lbs) results in anywhere between 10-13. 4 mpg. My mileage increase didn't occur overnight, and I truly believe adding the Stanadyne to each tank helped my injectors/fuel efficiency gradually improve to its current state. Also, I just turned over 44K on my truck, so engine break-in may be responsible for some mileage gain too...