The diff is Dana 30, and I believe there are two different carrier sizes. You can probably get some good advice on what ring + pinion swaps will fit within your existing carrier at an off-road shop, as Jeeps for years used the Dana 30 for the front axle.

Fuel mileage will be pretty much a wash, IMO. Tire pressure and driving style can easily mask the difference that the axle ratio will make.

Be aware that if your car has ABS, you need to swap in an ABS-equipped diff if you expect the system to work - ABS cars have a different sensor and tone ring to pick up wheel speed. You'll also need a different speedometer to account for the tone ring sensor speed as well.