Fuel efficiency

Where are you running? Out west? midwest?
Mountains make a big difference.
Are you on interstates most of the time or secondary roads?
How many miles are you trying to run per week?
Are you up north more of the time or down south?
(how much do you need to idle? or do you plan on an APU)
You see, there isn't a pat answer for everyone. It really depends on your business model.
If you're running between SoCal and Dallas most of the time and I'm running the PNW, there's a difference. I have a lot more mountains to deal with.

 

I cruise at 66 when I'm 30,000# or more which is 85% of the time. Anything less Im up to 67-68. And of course 55 in CALI

 

450 HP, 13 or 18 SPD, geared for 60 mph.

 

If you're running mostly interstates, I'd go for 450hp, 13spd and a bit tall in the gears since you're running out west.
I'm assuming you will run 65-68 if you're looking for mileage.
3:15- 3:50 or so.
You should be looking for 1200-1300 RPM at your higher end speeds...68or 70.
You can drop the HP a bit...425-400 if you want, but you'll be struggling on the mountains.
If you were running off on secondaries a lot, I might agree with the 18, but the extra shifting will cost you fuel and it's not a factor when running the interstates.
Just my opinion. See what other's think.

 

What's heavy? 80,000 120,000 150,000? I'm guessing 80. Gear for 1300-1400 rpm at the speed you want to run. get an 18 spd with 2.64 gears and then run in 16th, unless youre in a hurry then use 17 and 18. I'd stay 2003 or older, free flow exhaust, ECM tune, fuel cooler, fleet air filter(foam) if you have the small under hood filter

 

2bit hit where I was going. I like the 18 with 2.64 or 2.79 also. Run in direct drive (16th) which is 3- 4% more efficient than running in OD (per the senior engineer at Eaton). There is a loss of 60-80 hp at the rear wheels running in OD compared to direct. With low pro 22.5 rubber, 1400 rpm is 62 and 1500 is 65 with 2.64 in 16th. When you are light, no wind, flatter ground, and want to run a little quicker, then move on up to 17th. 1400 in 17th would put you near 70 mph. Not as much hp loss to rear wheels running in 1st OD (17th) than top OD (18th). In 16th direct, the tranny will run much cooler than in OD. Startability is not a problem unless maybe you are trying to pull out of a wet cornfield grossing 95K. Not every application can do the 18/2.64 thing, but 90% of what goes on in OTR trucking can. And if, God forbid, the nanny state decides to limit all trucks to 65 or 68 mph, this would still be an ideal setup. And go with as much HP as you can. Sounds goofy, but driven properly, a more powerful engine can actually get better mpg. Key here is "driven properly". With more power comes more responsibility.

Everything else 2bit said has some merit also. I would also add, do similar to what I did, if you can. Spec out a new glider truck and put in a rebuilt pre-emissions engine. Won't work for CA, but everywhere else is fine. And you will save a lot of cash over any other new production truck.... like 30% savings over another new comparably spec'd truck and a lot lower lifetime operation cost.

A truck, spec'd right and driven right, can get you into consistent 7+ mpg average territory all the time, even when running heavy. Plenty of owners have proven it. You do not have to drive like grandpa either. Driven right means using all the power and flexibility wisely. Stuff like staying off the cruise in hilly areas, progressive shifting, backing off power when cresting over a hill and letting gravity and little pedal give you speed back on the down side, actually using the turbo boost and pyrometer gauges to run engine most efficiently, etc.