Not only was your engine 'acting' smaller due to the lower air density, but also your Pathfinder was acting like it
had a virtual smaller total frontal area at elevation in CO as compared to driving the same speed in TX.
And of course the faster you go, the more that frontal area x Cd x Air Density matters.
****
I've always had an interest in MPG/efficiency. Once you spend just a little time rooting around
in the MPG rabbit hole, the fact that rolling resistance is a constant, but that wind resistance
(ie: punching a hole in the atmosphere) goes up with the
square of your speed, this means my
preferred method of rapid automotive travel directly affects my wallet. :-(
Of all the wind resistance graphs out there, I find the following graph to be the most helpful to me:
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(Compare the total resistance at ~45mph {~350 Newtons} vs 90mph {~2200 Newtons} Speed thrills...but also sux the gas!)
I like to use the following curves when trying to map a real-world 20 or 30 mpg vehicle to fuel consumption at speed.
(Note: For accuracy, you have to include any headwind
in addition to your ground speed in order to find the
most accurate spot on the curve below. (If you've ever headed West and pushed all day against a steady 20+ mph
headwind you know the difference it can make in a tank of gas.)
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A thumbnail sketch of what a given MPG # for a tank of gas represents would include:
* Wind Resistance x coefficient of drag for your ride (see squared resistance curves above)
* Rolling Resistance (Tire inflation = how much energy lost to flexing tire sidewalls + contact patch friction, size & shape)
* Pumping Losses (the bigger the motor x the higher the cruise vacuum {the worser} x the more Revolutions Per Mile {the worser}
* Parasitic losses in drivetrain: Power consumed to drive water pump & fan, stuff on serpentine belt, engine oil pump, auto trans ATF pump, & power through 90° angle in diff)
* Total number of changes in vehicle speed per tank.
* Total vehicle weight vs number of accelerations (more work performed per mile driven + 4 tires acting as large flywheels consuming energy with
every speed change.)
* Total Number of Elevation Changes per tank (!)
****
Given all of the above, if I wanted to post a defensible hero MPG number for my Built to Work big block chore truck,
this is what I would do:
1) Tires aired up towards the max rating. (Temporarily trading long term goal of even wear across the tires for min rolling resistance.)
1) Engine warmed up, fill up tank (slowly + 3 clicks) at gas station located on Highway 36 at the Colorado/Kansas border.
2) Head East, come up to speed once, and drive at the Rt. 36 speed limit, taking advantage of any prevailing tailwind I might encounter.
3) ~24 gallons later, stop driving smoothly, pull over to refill. (slowly + 3 clicks) Note: No hypermiling/getting in the way of other folks on a mission.
4) Post hero MPG for the beast. (And instantly lose credibility with some because they've never seen anything close to this # in a similarly equipped GMT400.)
I know, all of the above is concentrated boredom when gas is hovering at $2.00/gallon. But as gas prices are closing in
on $4/gallon yet again, I offer the above as food for thought. And I will bet a dollar that
@Nick88 has honed his GMT400
supercruise skills and has figured out how to minimize the effects of those big tires acting like 4 large flywheels that resist
changes in speed.
It's quite possible to achieve > factory MPG ratings. But the driver has to work within the confines of the 7 variables listed above. And that same driver
will never get the KS Rt. 36 nonstop MPG on nearby I-70 at 75-80mph heading westbound, or when heavy pedal action during the cut & thrust I see when
folks are rushing to/from work, or driving up/down the mountains roads of Vermont.
Given all of the above, I hope that Nick88 and others will continue to share what they've been able to eke out
of their GMT400 vehicles. FWIW I've driven everything from 70+ mpg 2-seater aluminum Honda Insights to
a lean-burn Civic VX on the far side of 60mpg...all the way to the chore truck, so from firsthand experience
I am comfortable to share the above.
I'm all about the data. Together let's see what we can discover about the GMT400/MPG calculus.
EDIT: Years ago I use to traverse Kansas on I-70 with the goal of minimizing the time spent doing so.
Oh so boring. And then there came a time where I just didn't want to face the sameness of that experience
yet again, so this is how I discovered the Rt. 36 experience. Sure, it takes longer in terms of wall-clock time,
but it felt a lot shorter, and I enjoyed the little towns I drove through. As a matter of fact, on the rare occasions
that I get to indulge in a road trip I almost never use the Interstate system anymore. No regrets. :0)
Cheers --
