The Dreaded P0171

Probably a super-duper-long-shot, but do you use a fuel injector cleaner as a regular part of your maintenance? You could have a partially clogged fuel injector.

I poor a bottle in every 5,000 miles when the fuel level is very low and drive it to almost empty. The difference in performance is small, but still noticeable. Do this for all my cars. The STP and Gumont brands work the best in my experience, but I tend to buy whatever is cheapest in bulk. The Lucas Oil fuel injector cleaner doesn't do much, but they do make great oil products though.