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What trans do you have? A 700r4 would be better if you don't already have one. Two of Four wheel drive? What rear gear do you have?
Being a 3/4 ton I'd be willing to bet its a th400 n it with at least 3:73 gears maybe even 4:11 or 4:56. My 3/4 tons all came w 4:10s. The 400 trans is parasitic, but very stout. Prob with it is you've got a 305 in it. A 700r4 would definately help, but do you just use it as a driver or to you pull/haul much with it?
Lol, there is 20-30 miles between suburbs out here.
From your original post you mentioned "just a couple of suburbs away. Are you driving Highway or stop light to stop light?If stop light to stop light, your mileage is not going to jump very much, even with a 4 speed tranny like the 700r4. That one's real benefit is overdrive at highway speeds....