I never thought I would see "fuel mileage" and "Rockwells" used in the same post... what next, who makes the best aggressive highway tires to use on Rockwells? :lol:
Most people with Rockwells run about a 44 inch tire, and then the gear ratio is what it should be for a tire of that size. If you are running 40's, 42,'s 44's, or anything bigger... fuel mileage shouldn't even be in your vocabulary at that point. You either want good gas mileage or you want a big truck on Rocks. You can't have both, and usually you can't even have a stock truck with good mileage. I don't see why you are even worried about that.
Rockwell 2.5 tons have 6.72 gearing. GMC 2.5 tons have 6.17 gearing. The Rockwells are the top loaders and the GMCs have a front load 3rd member (like a Ford 9"). There is much more available for the Rockwells as far as aftermarket parts. I don't think any of them came with lockers, but a Detroit can be added. It fits in place of the spider gears and reuses the case just like a 14BFF does. A very good source for aftermarket parts is Ouverson Engineering. They make billet shafts in stock or 2" sizes, lockers, spools, hubs, (all with alloy material), and a sweet 8 lug conversion hub that allows use of an 8 x 6.5 lug pattern.
We put a Super Duty with a PSD on Rockwells and 49s. If I recall it gets about 12 MPG. Not bad for a truck that size. I think properly matched with correct tire sizes, the mileage can be decent, but overall it's just the axle and size you'd want and not worry about mileage.
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