I own a 99 cab and chasis with the 5.9 diesel, 5 speed with 4.10 rear end. I find myself doing more and more highway driving without a load these days. I'm looking for some options to gain mpg's on the highway. I've heard that there is a elec. overdrive that can be added to the tranny to get that highway gearing. I've looked into changing the gearing but the carrier would have to be changed to get the 3.55 gearing. It being a cab and chasis the rear end is narrower than a standard dually so the rearend swap from the junkyard is virtually impossible. The local gear guy quoted me anywhere from 1500 to 2500 to change rear end gear. So for that type of money I'm looking for options. I like the elec. overdrive because it will allow me to keep the pulling of the 4.10 and still gain those highway gears. Any input is appreciated.......

 

For that kind of money I'd search low and high for a set of rear ends. When I swapped mine I found a set from a C&C ironically enough for way less than what I was quoted to swap gears... They are out there, just gotta look, will probably have to travel.

BTW is it a 4x4 truck? Was 1500-2500 for both diffs?

-J

 

It's a 2 wheel drive. The guy told me that the carrier for the 4.10 will not fit the one for the 3.55. That's the reason for the high price. A normal swap would be 300- 500$ if the carrier wouldn't have to be changed.

 

Are you already running 235-85/R16 tires? If not, you're most likely running a smaller diameter tire, and going to the taller tire will be the simplest route for better mileage.

 

Find a different shop. I found a Dana carrier for $200, a Detroit locker for $540, and an ARB for under $1k. So tacking on $1500 because he has to change the carrier is ridiculous.

 

His is correct, 4:10 and up is a different differential case than the 3.73 and down.

See page 5
http://www2.dana.com/pdf/XGI.pdf

Your truck being a narrow dually will greatly decrease chances of finding a complete rear in the salvage yard, but don't let that inhibit you from searching.
If you find one it will likely have been "very used" from commercial use and should probably be rebuilt. Consider a 3:31 ratio if that is the case.

The other option is a Gear Vendors overdrive which is outrageously expensive and also have reliability issues behind the Cummins that Gear Vendors won't tell you about. The Gear Vendors also requires oil to be changed every 5000 miles, a maintenance headache.

 

midnite hit the nail head.

 

Respectfully ......... "tall tires" are not a practical mileage solution for a truck equipped with a dually rearend.

Too big a tire, the sidewalls rub together causing sidewall wear and they also annoyingly squeak on uneven roads.

Also, most cab chassis body designs fight to keep the deck height as low as possible, "tall tires" will likely cause interference problems.

Wider than stock "tall tires" decrease traction in duallys which by their nature already compromise traction.
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Ghenges, we're not talking monster mudders here. I mentioned 235-85/R16s because duallies have been running them from the factory for years. It's one of the two common tire sizes for duallies, the other being 215-85/R16s.

Given that, I don't really understand why you're throwing up a flag that they would not be a practical mileage solution. They would be an entirely practical mileage solution.