01 excursion fuel question

[EXOWNZU]

I have a 01 Excursion. Far as mods go, a turbo back 4" exhaust with muffler delete, Hypertech stage 3 programmer-on stage 3 setting, AEM brute force intake. Leveling kit and 35's. There are no check engine lights, the oil and all filters are changed to Ford specs, and intake filter is cleaned on a regular basis. Fuel is bought at a station that sells tons of diesel on a daily basis so no worries of getting contaminated fuel from sitting for so long. Figured I'd give that little info to help with any possible things anyone might beable to come up with.
So my question is, during the summer I was getting about 950 miles till empty on the meter on every fill up. Lately with every fill up it has been getting less and less until now i'm down to about 750 miles till empty. In accordance, I was getting roughly 17.6mpg per tank and now i'm down to 15mpg per tank. I understand that it doesn't add up when you mulitply, but I never expect to get 950 miles out of a tank, but that much of a dramatic drop in the intial reading is kinda bugging me. I'm new to diesel so I don't know if this has to do with the weather getting cooler or what not. Maybe a stupid idea, but it's the only thing I can think of as there are no visible fuel leaks and I don't drive like a maniac. Any help is appreciated, thanks in advance. Kellen.

[Raul@Stealth Automotive]

maybe winter blend fuel? i forget when they change it but you do see less MPG during the winter then in the summer because of the additive they add...

-Raul

[EXOWNZU]

Thanks, I didn't think of that. I'll have to ask when I fill up again.