How about a gear vendors over/under drive? I know they're expensive, but it would give you that extra drop in RPM's at cruising speed.

 

Well, Im running a manual 6 speed with a granny gear so I would be making my granny gear 1st gear and 6th an over drive. I always start off in 2nd so I would just be using first.

 

My pop has a 11' cummins G56 with 3.42's. Granted he has 305 tires but the cruising speed is a nice. Your window sticker would show what gears your truck came with.

Point being, even if you have 3.73's going to a 3.08 is going to be quite a drop and may drop you out of that 1800-2200 sweet spot on the highway. Just my .02.

 

With a 48re your cruising speed at 75 mph with 3.08's gives you 1700 rpm which is ideal. It is right at TQ peak.

With the NV5600 manual, your 1700 rpm with 3.08's gives 1700 rpm at 71 mph.....so either way, unless you plan on gaining tire size....use 3.08's.

If you ever level it with 35's the stock 3.54's work great. I cruise at 1700 at 68 mph.

 

I'm in the same situation. I always start off in second gear, so I had the same idea as you. With 3.55's I turn about 20-2100 RPM at 65mph, and I could easily drop that down to 16-1700, with absolutely no problems, especially in my setup and what I use my truck for. That, along with advanced timing, and I think I could hit 35mpg on the highway. I wouldn't even consider 2.73 gears; thats way too high, and I think it would make fifth gear pretty much worthless. My big decision was between 3.08's and 3.27's.

Your G56 tranny behind the 5.9 has a .79 OD ratio, behind the 6.7 the G56 had a .74 OD ratio.

For you: 3.08 gear ratio, 32" tires, .79 OD ratio, and 65mph= 1661 RPM, about right.

Guys, doing the math is better than guesswork.

 

There you have it. Drop 3.08's in it and report back. My bet.. Trip mileage may be better. Around town driving will be worse.

 

Hey tanks! That's exactly what I needed to know. I'll put up my results when I get them. Right now I'm getting 20 mpg. Will see what it goes to when I'm done.

 

Not a problem

It really just depends on what you want to use your truck for. If you drive primarily highway miles, without a lot of stop and go, it would benefit you. But I feel the same way about town driving.

 

Ok so I didnt end up doing it and heres why. First the loss of power would be too much for my taste. Second the transmission shop told me that they didnt think gears that low in my truck would be good for it. The torque would shorten the life of the rear end and probably the tranny too. And the diesel shop I talked too in texas said that I would have to reprogram my tranny to adjust my speedometer.
Too many issues for the proposed gains. That and I cant find anywhere online where anyone does this whick probably means its a bad idea.

 

Exactly. Here's another thought: When fuel mileage becomes a big issue with your 3/4 ton diesel truck, either it's just not running right or you might be driving the wrong vehicle.