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Re: C30 dually rear diff gearing options help please
I have the 454 and 3.21 rear gear with the SM465 manual transmission in my 1984 C2500 from the factory. It does decent on the highway (about 13 mpg) even with that non-overdrive 4th gear. Difference being though is the transmission has a granny gear so it can get away with having a taller (lower numerical) rear gear and still tow just fine.
Had a 1975 with a 350 and TH350 transmission and 4.10 rear gears that got worse mileage at about 11 mpg highway at about 70-75ish mph. This led me to swap the transmission for a 700R4. That overdrive gear brought my highway mileage up to 14-15 mpg.
My suggestion would be to find a 4L80 and swap that in. Similar basic construction to the TH400 with the planetary gearset, but with the addition of an overdrive gear.
If you can get away with it cost wise, drop that rear gear down to a 4.10 or 3.73 as well and you will see an increase in fuel mileage.
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