I had a Gear Vendors overdrive on my Corvette. It dropped my rpm's about 500. I think they build them for mdt's (medium duty trucks like a 4700). You could check with them. Gear Vendors under/overdrive transmissions the most awarded auxiliary transmissions.
I think it will cost you about $2500. Might want to see how much to put different gears, or different rear end, in. Can you put larger tires/wheels on? That would lower your rpm's too, wouldn't it? What trans is in it now? Maybe there are other transmissions that might have different gear ratios that might work better? Weren't most of those 4700's built for delivery around town so not geared for highway cruising? They might have had other transmissions that weren't as popular but would work better?
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'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio