Loss of fuel mileage

Make sure all your filters are clean and clear, if you leave your truck idling for long periods of time it can contribute alot to bad mileage. Because it gunks up the fuel system. Sometimes it can be just as simple as an oil change though. It's surprising how bad a mileage you can get when your oils ready to be changed.

engine codes may tell something if the check engine is on, sensors that effect fuel delivery can deify the reason they were there from the start,, check the air filter for free flow, also, change fuel brand

Do you mean from 12mpg to 8.7mpg..... Or to 8.7 l as in liters...... Could be you changed the units from imperial to metric. Just reset them to imperial checking in owners manual.

12mpg to 8.7 mpg do you think it could be a vacume leak it runs a little ruogh