Probably does make the most fiscal sense to consider this "when the current one dies" or if you can find a willing buyer for it at a good price. Even used, a Fit isn't going to be cheap, usually.
For the one you have:
Depending what you do with the Jeep and what you have on it now for tires, you might find that a set of "boring street tires" preferably bought with boring rims so you can swap them if you do take it offroad or need serious snow action sometimes may help your milage a lot. Aggressive and wide tires can really smack the mileage on the road. Boring tires and rims can sometimes be had cheap on craigslist from people seeking ever more extreme tires and rims for offroading. Having the right tire pressure also helps, if you haven't checked it recently.
And, the way you drive also can have a huge effect on milage - costs nothing more than being calmer and planning further ahead so everything is smooth and nothing requires "stomping" on any of the pedals. Or you can invest a few bucks in an aftermarket mileage monitor if you don't have one built in (use the built in one if you have one.) Either can help you to see where your driving style might be affecting your milage, and no new car is required.
For the next one:
I'll add to the Fit praise - it's a huge small car if you don't need to haul too many people too. And unlike the Saturn and VW we had in between it and the previous Honda, it's, well, a Honda. There's more than gas to save money on...those things wanted new brakes every couple of years, among other problems. I don't know how yours is treating you but 3 people I've known with Jeeps over the past few years have also found them expensive to maintain in running order. A couple of others seem happy though.
Have someone you know who owns a Fit come by with it and drop the back seats - perhaps even test-fit your gear in there - you'd certainly want to do that before buying one, anyway. Be aware that there are slight differences in the different years as to the exact dimensions/shape of the back.
If you are hauling both people and gear (as with Kentucky Ryan's case above) a trailer can be hugely efficient - put all the gear in trailer, put the people in the Jeep, take one car. But, finding a good one at a good price takes time, usually.