The safety standards for the vehicles have really upped their weight and the pollution standards for the engines have really decreased their efficiency. Not to mention, mandated ethanol and other "oxygenating" additives to the fuel decreases the mileage they can provide. In winter, most areas have mandated blends for the weather conditions that increases the ethanol and other additives even more and thus the mileage goes down even more.
Newer technology, because of Mileage mandates are increasing their mileage even more, as well as the price of the vehicles.
I think the current year F-150 (maybe next years) have an Aluminum Body to combat the growth in weight of the vehicles from safety standards, and it barely any savings in weight at a great increase in cost, but the bar is set so high by all the different mandates, that is what they forced into with every little bit that helps being needed. As well, its arguable that the car manufacturer's learning the expertise to build aluminum bodies will benefit them and all in the future.
But for your Commander, the EPA has changed its test for MPG ratings to be more reflective of actual driving conditions, so your mileage may vary disclaimer of the past, varies even less today. BUT, you still have to drive like a little old lady to get the same as the EPA rated MPG. As well, if you're stuck in heavy traffic (doesn't has to be a traffic jam) there is no helping the mileage being lower than advertised.
Things to improve mileage, other than driving like a little old lady:
*You've already mentioned fresh spark plugs.
*Don't forget a clean Air Filter.
*Don't forget your ignition cables (some years Commander had them on their engines, some do NOT, having all coil over plug.) But, ignition cables, if you have them, do NOT last the life the vehicle, they degrade internally over time and with use and cut down on ignition energy. (there is a test for them in the FSM).
*Clean Fuel Injectors (a little fuel injector cleaner can't hurt once in a while) Chevron *Techron is considered a good one, there are more than a few that are worthless and nothing more than some alcohol in a bottle.
*Tires inflated to the proper pressure (higher gives better mileage). TPMS only tells you your pressure is getting low enough to be dangerous. You still have to check it and fill it to the proper pressure.
*Checking for stored fault codes and correcting any codes you find.
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