Seems like you would have to lose efficiency and mpgs the higher you go due to less oxygen

 

The ratio of oxygen to nitrogen is the same but it is just less dense. The turbocharger will make up for some of the loss in density.

 

Gunracer1 says better mileage. Maybe less power equates to staying under 65 mph on the Hwy??

Gear Poet says altitude hurts mileage. Since truck weight doesn't change with altitude...and there is less air...lower mileage makes sense also??

Signed, Confused.

 

As I mentioned I live at 6k just outside of the Denver area. I travel to south denver to work and it is about an 18 mile trip each way. Driving to work on a mixed tank of freeway and city I get about 18mpg. I drive from here to Cortez (375 miles) in the southwest corner of the state frequently over several passes that are over 10k driving a bit over the limit which for the most part is 65 but as low as 50 up down hills sharp turns with many grades of 6 or 7% for 8-12 miles at a time etc at an average elevation of about 8k I have gotten as high as 20.5mpg. but average 19.5-20mpg unloaded. Both above examples are if I am somewhat careful with my right foot which I normally am. I cant compare that mileage to sea level as my truck has only been there once and I was hauling a trailer of about 4k. I would be hard pressed to believe that I would get much better mileage than what I am geting here at sea level but some improvement would seem logical. I can definitely feel the power difference well at least the turbo lag is less and responsiveness is better at sea level.

 

If you can do that mileage in the mountains....with a 4x4, DTT auto...than I would say Mike (Gunracer1) is right!
My best with similar truck is 17 mpg (50/50 driving), and maybe 19 on a trip!

RJ

 

I suspect a lot of it has to do with speed. I sometimes have to drive to Hays Kansas (1100')and back in the same day which is about 325miles each way. I never get as good a mileage going down or coming back as I do here. However I also am running 80+ mph on the freeway and there is usually wind. Seems on the way down the wind is from the east and the way back it shifts direction and is from the west. I never seem to get a tailwind for that trip either direction.

 

Some clarification:

All equipment the same, except for a turbo change-over.... all kinds of driving, but mostly empty highway....

--------------------------------

HX35 stock turbo (2003):
Seven months in the Roanoke VA/Bluefield WV area:
19.8 mpg avg

Five months in the Salt Lake City UT/Elko NV/Steamboat Spring, CO area: 17.1 mpg avg

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HX40/35 hybrid (2004):
Seven months in the Roanoke VA/Bluefield WV area: 20.1 mpg avg

Five months in the Salt Lake City UT/Elko NV/Steamboat Spring, CO area: 18.9 mpg avg

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I noticed the following:

- EGT's are generally 100-200*F higher at higher altitudes, and of longer duration.

- Smoke control is tougher at higher altitudes.

- Turbo spool-up is slower at higher altitudes.

The HX40/35 made a lot of difference in EGT management at higher altitude. It made a small difference in smoke control, and no difference in turbo spool-up, as best as I could tell. The 40/35 made more difference in mpg at higher altitudes than at lower altitudes -- I'd thought because the larger turbo impeller was capable of compensating for the thinner oxygen content at higher altitudes, by moving more air.