Just a thought, but could the milage variance be due the difference in winter blends. I would think the various companies that blend the winter fuel do it different and use different additives. Could that be part of the cause?

 

Yes blending for winter/summer is part of it, but the other is blending above spec fuel with below spec to achieve a minimum, not necessarily the at or slightly above min spec. The blending can have wide ranging results based on what improvers are added at a refinery and then again at distribution terminals. Depends on the quality control and chain of custody communications and requirements. Some are excellent, some are not.

 

Very pleased with the milage after the TSB.
Was avg. 14.5 mpg in town
now avg. 16.5 mpg in town
Driving in the mountains of Colorado at 65mph, 18.5 mpg

I did have one trip (200miles) that was pretty mild with just a few stops, but speeds ranged from 55 to 65mph, but mostly around 55mph. My milage was 26.5 mpg on the overhead for the round-trip. Couldn't believe it. That tank averaged a hare over 19mpg because it was coupled with about 300 miles of some bad city driving. Had I hand-calculated that trip (filled after the trip), I think it would have had consistant readings.

 

Easy, you've got a 4.10 rear gear... Even at 70 mph your spinning a lot of rpm's. Also the "winter blend fuel". Even if the temps havent dropped where your at the stations may be using the winter blend fuel in which they add somthing or another to keep the fuel from gelling and EVERYBODY with a diesel sees a few less mpg running this...

If I fill the tank, pull out on the highway and run for hours at 70 - 75 mph I get about 16.1 - 16.8 on the computer... If I drive around town and keep it close to 45 mph I can get it up to 22 or even 24!

The 4.10 gears arent as big a differance as MANY here think from the 3.73 rear. They WILL cost you about 1.2 mpg overall(I know this from experience). But on a good long hill you'll pull away from the 3.73 truck every time. Several truck lengths with the same fifth wheel behind each truck (do it every time and my buddys truck is a SRW... so he has the weight advantage if anything).

As far as the mileage getting better with time... Very wishfull thought according to every vehicle I've ever owned! This truck is number 39 for me and I've NEVER seen mileage improve because of break in yet! One of my customers is in the GM R&D prototype lab., I asked every engineer in the room is they ever heard of this "mileage increase"... They laughed.

 

Good call BDude, 4.10's will do it.

Wind will affect mileage alot more than 1-2 mph also. Depends on how fast you are going and how hard the wind is blowing. I've seen a 40 MPH headwind drop mine by 7 MPG towing a cattle trailer.

 

Like the guys already mentioned... Verify what the computer is telling you! If you modify ANYTHING the computer will read wrong. Its calibrated to the stock truck. Mine has been off by nearly 2 mpg since removeing the muffler, and just the intake stuff threw it off by 1 + mpg. (the computer has been reading on the low side)