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I would suggest checking out some local salvage yards for rangers/mazda pickups for the complete rear end. That way you could just unbolt and bolt the other one in. It may be the same or cheaper when you add up the cost of new gears and having it done if you cant do it your self. I am trying to do this for my 92 ranger 4.0 but I want to go from 3.27 to 3.55 or 3.73.
3:08 ain't much of a gear for a truck, if you did 75% of your driving on the freeway, it may help, but in stop, and go driving, you'll be giving it more throttle to get the same power level. Zack (RANGER PILOT) has 3:08's, and a manual, and he has said that he only gets 16 mpg. I have a 4x4 with 4:10's, a manual trans, and a 4.0 sohc, when it was stock, I was getting 18 mpg around town. These over head cam engines do seem to get better mileage. I had a 2000 ranger 4x4, 4.0, automatic, with 4:10's, and I only got 14-16 in town. Maybe some more people will jump in that are running 3:08's, and let you know what thier getting, I don't think there's much difference, I could be wrong!
I wouldn't go any lower than a 3.27:1 if one is available for the 7.5". A 3.08 is going to leave you in lower gears too long until you get up to a good highway speed. If most of your miles involves long stretches of higway driving, you might see some benefits to the 3.08s. Unless you can set up the gears yourself, you're probably better off finding a truck that has the ratio you're looking for and just swap out the entire rear end. Labor to swap gears can run anywhere between $150-400 depending on who you know and the quality of the job.
Those taller gears are better reserved for torquey V6s or V8 engines. A 3.0L doesn't exactly fit that bill.