". . So my take is that your driving habits affect the on-board computer which affects your gas mileage."
The "modern" version of a vacuum gauge to monitor efficiency is an Ultragauge or Scangauge. Driver feedback cortesy of an OBD reader. Well worth it. MPG is about the overall annual
average, and it is the percentage increase to that number made by paying attention to the tenths that pays cash money.
Not to mention knowing exactly how to hit a grade that speed and fuel burn at the crest can be predicted by the driver . . . I may drive 12k miles/month, but knowing how to "run" a trip as part of trip-planning means details like this are part of figuring how to best use allowable hours (Federal Rules) on an outbound leg of a roundtrip. Seconds, minutes and days, it all comes together intuitively over time.
I can choose to drop gears or not on some ascents, but engine supplier CUMMINS says to allow that 15L motor to lug down to 1200 rpm before the downshift. It "may" be faster to drop a gear at 1500
, but the motor (relatively) screams.
Best use of fuel is also best use of power which is less engine wear & tear. And the trans. An average of 12 over 11 is a good goal as it is close to 10%. The skill acquisition and vehicle prep will pay off.
A very careful reading of the Operators Manual may reveal several strategies. We've found that re-engaging the cruise control at 60-mph will take us back to the governed 68 faster than keeping the throttle punched. The computers really are smarter (rather, with full control of parameters, they can allow more throttle, so to speak), so experiment.
Hopefully, no one here is climbing grades that slow big trucks significantly much faster than they. Dropping to 50-55 (or the minimum 45) will keep one out of trouble if the need to brake arises (fools changing lanes around one another; see this far too much). Slack in the rig lash-up is always a problem.
Best to be able to accelerate away
as this is the only condition where the lash-up is taut. The rule of thumb is to always climb a grade in a gear where one can still accelerate. It's usually the best with which to make the descent.
Learning full manual control of an automatic is how I started over forty years ago. It should be much more fun with todays greater number of gears.
Where safety and fuel efficiency meet is also the numerical baseline of the rig. TT axle alignment and bearing pre-set. Brake adjustment (no drag). Same for the TV. Make no assumptions they are correct unless verified. Same for using a certifed scale to set up the WDH and to get TV tire pressure dead on. Tracking, weight balance, etc, are all part of best performance.
Those tenths of a mpg add up quickly. KENWORTH says, among other references, that steering corrections per 100-miles is measurable as to fuel efficiency. I've always favored "fingertip steering" for reduced stress if no other. Work toward that goal -- via gear and baseline -- and much of the rest falls into place.
And put the cruise control on ASAP. A setting a couple of mph below what one "thinks" is best usually makes up for a CC that is "aggressive" in climbing the lesser grades. And this DOES NOT change the trip time, so to speak. The big truck and engine makers all advise the use of CC. Where safety is first, MPG will follow. Conversely, having to hit the brakes hard is evidence of poor driving skill. There is room for improvement, drastically, if it's more than a couple of times in 10k miles.
(I can hear the complaints, "but most of my miles are solo, I only tow X-miles per year!" Yes, . . drive the solo miles as if hitched for braking distance is the number one lesson.)
FWIW, my folks second TV, an '87 EFI V8-454 Suburban, hauled their Silver Streak
in a combined rig of the same approximate weight. But at 6-8 mpg. The occasional 9.