Elevation and Mileage

[Gunzz]

Hey folks have a question that I hope someone can give me some insight on. First I am driving a 05 H2 and the only performance mod is a re calibrated computer. This was done with a Diablo Predator unit using the 87 octane setting.

Now here is my question. Recently we took a trip to the Pocono mountains. (Probably only 1500' elevation). During this trip I was averaging 15, YES 15 mpg in the mountains. As soon as I returned home (New Jersey near the shore) I went to back to my average 12.5 to 13 mpg. Of course this is driving conservatively. Weather, and temp is the same. So my question is why was I getting 15 up there, way above what I normally get? Do you think it is only because in the mountains you can do alot of coasting downhill? My thinking is that you also have to drive up hill alot also so that wouldn't be the reason.

I have repeated everything (no A/c, same weight, same passengers, same temps, same gas and full tank) and cannot duplicate. Someone give me some insight please.

Thanks Mike

[BlueHUMMERH2]

Were you reading the DIC calculation, or measuring the MPG directly? I don't think 1,500ft would make enough of a pressure difference to affect your MPG. Now 10,000 feet would definately have an impact.

[Escondido]

Not that 1500' is much elevation, but I thought elevation had a negative impact on mileage as the engine has to labor a little more. I thought some of the owners from Colorado had commented before on this topic.

In another post I just commented on our run to Colorado and we experienced better mileage (yes measured manually, not the computer) and it was through the passes.

Your experience was probably due to the less aggressive driving. We found that keeping it below 70 is the sweet spot for mileage. This keeps your avg rpm below 2k and really helps.

I think the talk for '08 is a new 6 speed transmission (new for H2) and I guess in theory that should allow it to run at lower RPMs at a speed more acceptable those of us with a lead foot.

[mdoyle]

At higher elevations the air is thinner and you'll have reduced volumetric efficiency, meaning each intake cycle will pack in less air and fuel mixture at ratio = less power.

The motor tends to run hotter with reduced VE and has more difficulty dissipating heat which will also hurt efficiency.

On the plus side, Hummers are aerodynamic like a brick and the thinner air might have helped reduce drag

[f5fstop]

A 350 hp engine, driven from 0 feet above sea level to 1000 feet will drop to 337 hp, thus affecting fuel mileage.

[CslRkH2]

I get about 13 mpg around Denver. When I travel to the midwest my mileage deteriorates to about 11. Before I moved here from the midwest I averaged about 16 in a GMC sierra and while out here visiting noticed I was getting about 19.

I do notice more power at lower elevations for certain.