I have a 2004 F150 and just had a new OEM engine put in. The gas mileage sucks. The mechanics aren't sure why but it was 17mpg at first then within 2 days it went down to 9.3mpg then after about 2 weeks it crept up to 10.5mpg

 

Did you do the break-in procedure?

 

the shop didn't give me any info on a Break-in Procedure. do you have that info? I did have the oil change at 500 miles, but as far as anything other than that, like speed/rpm variating.....nothing passed onto me.

 

From page 5 of your owners manual...
BREAKING-IN YOUR VEHICLE
Your vehicle does not need an extensive break-in. Try not to drive continuously at the same speed for the first 1,000 miles (1,600 km) of new vehicle operation. Vary your speed frequently in order to give the moving parts a chance to break in. Drive your new vehicle at least 500 miles (800 km) before towing a trailer. Additionally, during the first 500 miles (800 km) that you tow a trailer, do not drive over 50 mph (80 km/h) and do not make starts at full throttle. This style of driving will help the engine and other parts of your vehicle wear-in at the heavier loads. Do not add friction modifier compounds or special break-in oils during the first few thousand miles (kilometers) of operation, since these additives may prevent piston ring seating.

 

Ahh, thank you sir. Didn't even think about that.