quick mileage question
 

i was hauling a van ('98 1 ton 7.3 pwrstrk) full of people and a small trailer full of tools and luggage around the mountains of colorado. on the way to colorado from iowa, the mileage was only 13-14 mpg or so. once we got into the higher elevation, the mileage went way up to about 18 with the trailer and everything. without the trailer and just people, i was getting 20 mpg on flat land at 7500 feet elevation (San Luis Valley). i'm confused as to why the thinner air at the higher elevation got me better mileage? (without the trailer the mileage in iowa is only ever 15 max.
any thoughts would be awesome
thanks

I hear ya. I purchased my truck in Bozeman, Montana and got 17 mpg in Bozeman and have never got better than 16 here in Boise. I don't understand why it got better mileage at higher elevations?

It might have something to do with where you bought your fuel. I have found that if I get my fuel from a dealer near the freeway, who sells a few thousand gallons a day, I get much better mileage than if I filled up at a local station. I really don't think the higher elevation has anything to do with it. I would think you would gett beter mpg in a lower alt. with more dence air. But who knows

that's what i thought, it makes sense that lower elevation would give better mileage

On your better milage at higher alts, I also am a private pilot and the higher in altitude you go the more you have to lean the fuel because of less air [better milage] that is why jets fly at 45000 feet.

I might be going out on a limb here, but in addition to the leaner fuel delivery by the PCM due to the thinner air, it is also possible that the less dense air creates less drag on the vehicle as well equalling better fuel mileage?? I don't know?? any physics majors around :)

yes, the physics of that would be correct, but the overall drag difference wouldn't be a terrible lot. (i'm not a physics major, only engineering beginner)

I'm not sure how the "thinner air" argument stacks up for mileage. It makes sense, but how would you explain my best mileage driving on I-95 in the South near sea level? I got about 20 MPG, 2-3 MPG better than my typical mileage.

The theory of less dense air means leaning the fuel out which in turn creates better mileage makes sense, sort of. My question is this, if that was the case then wouldn't a person have a loss in power? A lot of folks are going to the aftermarket intakes to get more air in the motor, not less and claim mileage improvements. I thought aircraft fly at the higher altitudes because of the less dense air means less drag; less drag means less power needed to get to your destination. Now if the aircraft wants to haul a$$, they hit the deck and light the burners.......
I have no data to back this up, just a personal opinion. I think Cookie is the local jet engine mechanic, maybe he will chime in.