Welcome to TRS
I assume you mean Automatic since 5 speed manual should be getting about the best MPG from engine and weight of vehicle possible
1985 and up saw Ranger automatics get computer control, partial control at first, torque converter lock and Overdrive shifting(1987).
In 1995 they got full computer control for all shifting, the 4RXXE, XX was 44 or 55 and was engine/gear ratio related, transmission was the same.
These, A4LD and 4RxxE transmissions were all 4 speeds, 3 speeds with Overdrive
5R was 5 speed, 4 speeds with Overdrive
As far as MPG they would be the same for highway driving, 4 or 5 speed, because Overdrive ratio is the same.
Around town a 5 speed might help a little but wouldn't be by much, 1 MPG maybe.
Adding more gears between 1st and Overdrive is more about power, to try and keep engine in max torque range than about MPG.
Or in the case of pulling larger loads it is still about the torque range of engine.
People install 6 speeds for quickness not MPG
Semi's use multi speeds for pulling heavy loads because diesel engines have a narrow torque range
Look and see what rear axle ratio you have, look here:
http://www.therangerstation.com/tech_library/axle_codes.shtml
depending on the engine you have and what you need for driving you may have the wrong ratio
4.10(or higher) is great for pulling loads but not great MPG on the highway
3.45(or lower) is great for highway MPG but lacks power for towing/hauling
3.73 is the most common, OK for towing and OK MPG, but not the best for either