So what do you mean the "truck thought" that's the fuel mileage? I'm confused.. so since changing to the new air filter, the reading is now 8.5 mpg vs before with the stock system, it was reading 18.6 mpg?
You saying that the new air filter has confused the ECM, as to the true mileage? Surely the rolling weight of the trailer couldn't have killed the mileage that badly..climb a lot of hills in low gear?
Perhaps it's because you can't tow and run the tranny in OD. (Least you weren't supposed to with the 700R my truck has),
I could see where that would kill your mileage..