One of the biggest issues with the magnum engines is the intake plenum gasket.

When that starts to leak, the mileage goes down the toilet.

When I first bought my 92 Dakota with a 5,2 it got 9mpg. I did a tune up, thermostat, and temp sensor change, no better mpg.

Then I learned about the plenum gasket issue, sure enough that was the problem. Fixed that and I was averaging 17mpg with 95% highway driving, with 26" tall tires and 3.55 gears.

I now have 4.30 gears and the same tires, and still get 15mpg.