If you're uncertain if you're speedo gear has been changed, get one that fits your current setup. I put mine in when I realized that the previous owner hadn't after he lifted it and put on 33s. This will help you get a more accurate idea of what you has mileage is. After I put mine in, I'm in the neighborhood of 300 miles per tank. It's good enough for me. That's about the same total mileage as my STI I had before the Jeep.
Frankly though, don't worry so much about it. Enjoy the Jeep. It gets crap has mileage but makes up for that in fun points. Don't be too upset with guys telling you to just sell it. The long time guys here get tired of answering the question. It's been asked repeatedly and answered repeatedly.