Those little rangers are awesome, but there are a lot better options for a Cheap gas mileage vehicle. Personally I used to have a Hyundai Accent. I bought it for like $1500 and it was decent to drive and averaged around 33 MPG (more city than hwy). You can also get a Toyota Echo pretty cheap and they get of 40 MPG. There are others too, like the Geo Metro, Ford Aspire, but honestly they suck to drive and are kinda disposable cars. I mean if you are gunna buy a whole second vehicle you really have to make the gas mileage worth it.
Personaly I always advocate people to buy a motorcyle. You can get a decent bike like a Suzuki Marauder, Or Kawasaki Vulcan, etc etc for $1500 - $2000 and heck of a lot more fun to drive, cost about a quarter as much to insure, and will get close to 50 MPG.
But everything I do is on a budget. And the way I see it if the Ford Ranger got 26 MPG and your Jeep is 16, at $3.50 a gallon (assuming you drive the normal 12,000 miles a year and switched and put 100% on the truck) Means you would spend about $1200 a year less in gas. Of course you will drive you Jeep sometimes, but if you factor in the cost to register the truck and insure it, you will be lucky to save about $700 a year in gas. Pretty much no matter how cheap the truck is it will take you several years before you earn that back ......
Doing the same math with a motorcycle or car that gets 45 MPG means you will spend about $2100 less a year in gas minus registration and insurance, and in that case more like $1600 + a year savings. Meaning if you buy one for around $1500 it will come close to paying for itself in the first year.
Anyway just my 2 cents ... Ford Rangers are great trucks (ran 1 for a while and very useful little truck) but unless you need the truck aspect of it, its not really helping your budget. Plus these trucks tend to be a little more expensive since they are so popular with the work trades that do need the truck aspect.