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I have 2003 Kia Sedona 2.5 liter AT in good condition. I've replaced some of the old parts with new ones.

Both highway and city fuel consumption is bad compared to other cars with same engine displacement.

I'm wondering what makes the fuel economy so bad? Is it caused by dirty injectors? Bad ignition timing? Wrongly calculated flywheel?

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What old parts have been replaced, specifically? What fuel economy are you seeing, exactly? –  Mark Johnson Dec 10 '12 at 0:40
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2 Answers

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There are many factors which could influence your fuel consumption, namely, the design of the car itself - not much you can do about it. Are you getting what the EPA label said you would be getting?

Also, here are some things that will improve your fuel economy:

  • Use full synthetic oil and change at regular 5k mile intervals
  • Maintain correct tire pressure (look at the label inside the door jamb)
  • If you haven't replaced your fuel filter yet since 2003 - time to do that now
  • Buy some seafoam (or STP alternative) before you change your oil and add the full bottle to the crankcase and drive at least 50 miles with the seafoam in there - this will clean your engine
  • After each oil change, add some seafoam (or alternative) to your gas to clean the fuel injectors.
  • Check your alignment/tires. Bad alignment can cause your car to drag (also bad for the tires)
  • And lastly - drive smoothly, don't accelerate quickly, avoid braking and speeding up. Take it easy.
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You haven't mentioned how many miles per gallon you are getting but several things come to mind. How are you calculating milage? Fill the tank until the pump shuts off. Drive until 1/4 or less remains. Refill at the same station using the same pump if possible. Refill until the pump shuts off.Divide mileage by gallons used. Things that will cause a drastic drop in mileage. A brake that is dragging or not fully releasing. An automatic trans not shifting into overdrive. The speedometer may not be accurate so that you are actually going farther than you think. A fuel leak, although this should be obvious. Theft of your fuel.

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