One contributor to poor mileage is the AWD system. That means that both axles are engaged all of the time, which costs additional fuel. That's why lock-out hubs were made for part-time 4WD systems - to get the front wheels free-wheeling.

I don't know if you have a V8 in your Grand, but that will be another contributor to fuel costs. Admittedly, they're more fun, and I have v8 engines in both of my 4x4s.

Take BigJim's advice. I'd do it twice - once with mostly driving around town, and again sometime on a trip where most or all of the driving is freeway.

But, in the end, keep in mind that Jeeps were designed for fun and utility - not for good fuel mileage.