04' Columbia MPG estimate help...

5-6.5 mpg is a rough guestimate. Just remember. A Columbia is on the bottom end for creature comforts. It is the cheapest truck that Freightliner creates.

 

I have a 2000 Century with a C15. The Century is nearly the same as the Columbia, except for the hood and trim level. The Columbia is a little more areo than the Century.

When I first got my truck it was a dog on fuel economy. Just running bobtail I was lucky to see 8 mpg out of it. Now when I run bobtail (at 55) I expect to average over 12 in good weather. There were three major problems, the fan, the injectors and the horsepower.

The fan would come on and stay on. This cost me about .5 mpg. I had the fan repaired twice, but the problem would come back. Finally the bearings wore out to the point that it was non-repairable and the whole unit was replaced. I do little things now to try to keep the temps down to avoid the use of the fan. During the cooler months, I will turn the heater on while pulling a grade to help keep it cool. During the hotter months I will delay turning the AC on until I get up to speed. I've found that when running a grade that there is a sweet spot (around 40 mph) that maximizes the air flow thur the radiator hence the cooling to help keep the fan from coming on.

The engine was "thumpy" under load. It felt unbalanced and ran really hot. I had two different shops look at it and do injector cutout tests, but they said nothing was wrong. At 920k miles the engine sucked a valve, and I had it rebuilt and turned up to 500 hp. (they wouldn't go any higher because of the drive train not being rated for it). They were initially unable to get it to make 500, and had to really check the injectors. They found two that were cracked and unable to deliver a full shot of fuel under load. OTOH they worked just fine at idle or low load, which was why the cutout test wasn't finding them. What it meant was that the engine was trying to run under 4 cylinders, and those cylinders were getting very abused. I doubt that I was even able to get to 435hp. I wish I had followed my instincts earilier and replaced the old injectors with a rebuilt matched set. I might have avoided that rebuild.

Increasing the hp allowed me to stay in gear longer before having to downshift and run at a higher rpm. Generally the lower the rpm the better the efficiency. I've found that 1200 rpm works well for cruising, and 1400 works well for power.

If I had your Century I would probably do the following.
1. Wheel balancers (like Centramatics or BalanceMasters) on steers and drives. (.25 mpg gain.)
2. AirTabs (about a 5% increase, about .3 mpg)
3. ECM tune to increase hp and reset emission settings. I gained about 20% or more. As near as I can tell my 500hp tune is a pre emission 1999 tune. The tune can be a little pricey, but it does pay for itself.

4. I would really really think about changing the 3.73 gears to a lower number. I run 3.55 which work out great for me as I am a committed 55 mph driver, and that puts me at 1200 rpm at 55. (I drive mostly I5 in Oregon, a 55 max state for trucks.)

Once you start chasing mpg's you start doing a little marginal things to get to that next .1 improvement.

Currently my truck is in the shop having the muffler replaced from stock to a Walker Megaflow Noisebreaker. I have a turbo coming in from Mr. Haney, and I am pondering doing a manifold change.

 

The 3.73 rears are the big negative if you are going to be running mostly interstate. What size rubber?

 

I wouldnt let CAT do the tune. they messed up on mine and will probably on yours......... are you sure it is an 04 with an 02 engine.... i have an 04 with an 03 435mbn bridge motor...
what i did was,, i took the catalyc mufflers off... almost 1 mpg better
then retune my ecm from an mbn to an 6nz... i am averaging about 7 mpg.
also i do have a non wastegate BW turbo
if you want power go with a 7cz file which is 600 hp 2050 torque
if you want fuel mileage go with 6nz file 500-550 hp 1850 torque
i been experimenting with changing injector trims and fls/fts # and one thing i have noticed is if you open up the fuel to the engine the better mpg's you will get and lower gph usage when at an idle... right now i burn around .22 gph at an idle

 

In Cr Englnad the trucks I was driving standards were 500hp detroit 13 speed 3:58 rears = 7.2 -8.1 with tail wind on 80,000# connecticut to Washington.