Fellas,
I just bought a 2008 fx4 and it is only getting about 275 km to a tank. brutal. I have A T tires on and expect that in the winter I'll burn more fuel but this crazy. any ideas? I'm changing the plugs this week and putting in fuel line cleaner. I changed the air filter last week and made sure the tires were inflated proper... could it be a faulty O2 sensor?

 

Upstream O2 sensors should be changed at approx. 175,000 kilometers

When they get older the engine will run richer, spark plug tips should tell you if that is the problem.
If they are just light brown then O2s are not the issue
Blacker is Rich running

Whats the axle gearing in the truck?
4.10 doesn't get great MPG on highway

Check door sticker for that, read here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes

Also check stock tire size from that same sticker, speedo/odo is calibrated for those tires, and compare to your current tire size.

Larger diameter tires will make speedo and odo read lower, so you could be 300+km, if tires are larger than stock

Google: tire size and speedometer

You can calculate difference