Just bought a 2008 250 5.4 liter super duty truck with 410 gears and 4x4.
Gas mileage is around 3km per liter. And a bit less with towing. Iam towing a 16' enclosed trailer around 5000 lbs. And the truck seems to struggle to tow it. The rpms are higher than it should be when towing and not towing. The sparkplugs were bad and changed and so was air filter. That helped a bit. I was told to change tranny fluid and filter. Would this help? Any other suggestions. On the dash the computer is telling me iam getting around 29liters per 100 km but was at 35 before plugs were changed.
The reason for buying 250 was to tow a lot easier than my 1/2 ton Sierra. Which the gear ratio was a lot less but the engine was about the same.

 

I would have someone check if the cylinders were damaged by the sparkplug change. It doesn't take much porcelain left in a cylinder to damage the walls. A Ford scanner can do a relative compression test or you could do a leak down test.

 

Thanks for the reply. I could do that but the problem is a bit better once they did the change not worse. So not sure if that would help. But will ask. Thanks