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I have a 04 F250 with the 5.4 and 65k miles. The computer ised to say 9.7 mpg bow over the last 5 mounths it reads 7.2mpg. I have reset the computer but never figured it out on paper, what might vause the change?
But, to answer your original question, there are dozens of causes for a decline in fuel mileage.
Weak COP
Worn out spark plugs
Dirty MAF
Malfunctioning IAC
Dirty air filter
Catalytic converter getting clogged
PCV valve bad
Slipping transmission
Suspension lift
Tires with more rolling resistance
Tires not inflated properly
Front end alignment
Front 4 wheel drive hubs locked
Axle bearings worn
Gasoline with more alcohol
Larger tires
More junk in bed of truck
More junk in cab
Passenger weight gain
Weather
But, to answer your original question, there are dozens of causes for a decline in fuel mileage.
Weak COP
Worn out spark plugs
Dirty MAF
Malfunctioning IAC
Dirty air filter
Catalytic converter getting clogged
PCV valve bad
Slipping transmission
Suspension lift
Tires with more rolling resistance
Tires not inflated properly
Front end alignment
Front 4 wheel drive hubs locked
Axle bearings worn
Gasoline with more alcohol
Larger tires
More junk in bed of truck
More junk in cab Passenger weight gain
Weather
Good post Stuart lol. I saw a 2 mpg drop in mine at 40,000 miles. Turns out it was 2 bad coils and a bad plug. I calculate mine by hand on every tank and wouldn't trust the electronic one either.