'98 Ford Expedition XLT (5.4L)

Replacing an engine and tranny is very expensive. I would only do that if the old drive train was beyond repair. A new engine will require replacement/reprogramming of the ECU, and depending on how much has changed between years replacement of parts of the engine wire harness. If can turn into an involved project if you want to swap to a newer generation's engine. Other factors besides the engine also affect gas mileage, such as exhaust (old catalytic converters can become fouled or plugged and restrict exhaust), rear differential gearing, worn suspension and tire air pressure, worn torque converter clutch. At the end of the day, it's still an Expedition, they're not known for great gas mileage. If that's your chief concern, sell the old gas guzzler and buy something more gas efficient.

Thanks, KenF! That will help inform my decision. You're very right about your last point. My car currently gets around 10 city, maybe 12-13 HW, and as a college student who is swimming in disposable income, any MPG higher than those numbers would be welcomed. I'll look into those other factors, though, especially the cat. I don't think that has been replaced since my family purchased the vehicle over 20 years ago. I'm sure the suspension is affecting it significantly as well. That car has been through a lot of off-roading and pulling trailers.

It is not worth doing a conversion unless you have too much money and don't know what to do with it. Buy a new Expedition with all that loose cash and save the headaches!