I own a 2010 Ranger XL 2wd with a 2.3 DOHC and 5 speed. I have yet to take a long hghway trip with the Ranger, but in mixed commuting rush hour traffic, (50/50 city & freeway) I normally get between 23-26 MPG, and have gotten 29 MPG when a moderate freeway trip of 3 hours was added to the mix. A typical fuelup averages 56 Liters (14.8 US Gallons), and will normally be good for about 550 Kilometers (343 miles). I still have the window sticker from when I bought the truck new, it claimed 39 MPG highway, although I`m sure that was based on the older, larger Imperial Gallons Canada used to have before the Metric system took over in the mid 70`s. An Imperial Gallon is 4.54 Liters, compared to 3.78L for a US Gallon. My Ranger has 3.73 gears, but keep in mind the later Rangers have 15" wheels on the 4 cyl,`s where the older trucks were 14". My truck has a 200-250 lb fiberglass canopy, but the extra weight is likely offset by the aerodynamics, as I noticed no differance in mileage.
I agree with Rouge Wulffe, to "tall" a rearend gear is not good, I used to have a 1986 Ranger 2.3 5 speed, and with the 3.08 gears it had, 5th gear was only useable at 60 MPH or higher, and only on flat ground. Any hill at all would pull the engine down too much and require a downshift to 4th.