Gas & Mileage in the Rockies - Better?

[TeryT]

As we're toolin' around on vacation up here in the Rockies (trailer is parked), I've noticed better gas mileage. We did a trip from Gunnison down to Denver & back today, and got close to 22 MPG going down, and 21 climbing back up! (yes, calculated - not by the computer, which remains about 7% off, i.e., over-reporting mileage)

Now we obviously don't need to use the A/C as much here as in Arizona, but I don't seem to get this good a mileage in low altitude Arizona in the winter - usually no more than 19 or 20 at best. But again, when we go to higher altitudes in Arizona, say above 6-7,000 feet, the mileage improves.

I have also been using the 85 octane here, and see no issues with power, pinging, or (of course) mileage. Octane can be reduced in thinner air, due to not less knocking, so they offer the lower stuff.

My dad says that the better mileage is due to reduced resistance to the vehicle moving through thinner air. This makes sense, but I've also always heard that gasoline (non-turbo or supercharged) engines loose about 3% power for every thousand foot altitude gain. -- Perhaps newer computer controlled, fuel injected engines compensate for altitude better?

So what are some of the thoughts of people who operate at higher altitudes? What explains the better mileage? (shoot - maybe I'm just so relaxed on vacation I don't lead-foot it as much! LOL)

 

[Light_Mania]

Your improved mileage is definitely due to the reduced air density at altitude, which reduces wind resistance at a given speed. Air density decreases about 3% per thousand feet altitude gain. Gas mileage won't increase in direct proportion because rolling resistance remains the same, but you'll still get a noticeable increase in mileage at cruising speeds.

The fuel injection does do a good job of compensating for altitude by delivering less fuel. Yes you do lose power, but not combustion efficiency.

 

[RonMAIDEN]

Only highway driving mileage would be increased,around town it's not much different than our sea level buddies.
I do use 85 up here with no problems.

 

[03_4x4Runner]

On our trip this summer from Lake Havasu Az, up through the rockies (million dollar highway, through Gunnison, up to Wyoming etc.) I averaged 19.78 MPG calculated for 2400 miles. I also used 85 octane in the mountains with no problems. One thing that may help is that the speeds are slower in the mountains but there sure is a lot of up and down driving and a lot of curves.

 

[TeryT]

Your improved mileage is definitely due to the reduced air density at altitude, which reduces wind resistance at a given speed. Air density decreases about 3% per thousand feet altitude gain. Gas mileage won't increase in direct proportion because rolling resistance remains the same, but you'll still get a noticeable increase in mileage at cruising speeds.

The fuel injection does do a good job of compensating for altitude by delivering less fuel. Yes you do lose power, but not combustion efficiency.

Interesting that you loose air density and HP both at about 3% per thousand feet. I guess the edge then goes with the reduction in air density over the HP loss.

Pulling the trailer should be a little easier then. I've only got limited experience, but it seems the case that it will pull up that long grade coming out of Denver, easier than it will coming out of Phoenix - even though both are gaining several thousand feet. The other plus is that it's cooler on the tranny!

 

[Light_Mania]

A reduction in HP does not translate to a reduction in fuel efficiency. The fuel injection just serves up less fuel to compensate for the reduced mass of air (oxygen specifically). As far as what the combustion chamber sees, being at altitude is no different from being at sea level but with an incompletely opening throttle.

Due to the reduced air density, the transmission cooler and radiator is actually less efficient - there is less air mass to transfer heat to.

When climbing a steep grade, wind resistance is not going to be the limiting factor to your speed. You are lifting a mass against gravity, and at lower altitudes you will have more power to do this with so will be able to do it at higher speeds - so the climb out of Phoenix should seem easier than the climb out of Denver.

 

[TeryT]

I've actually seen this debated over on R.net a few times, and it never seems to come to a full conclusion.

 

[bonedoctor]

High altitude 4Runners

I live in Aspen, altitude 7900 ft and do all my driving around this altitude. If I go downvalley, the altitude drops maybe a 1000 ft, but if I go the other way, I'm always above 7900 ft and sometimes as high as 12,000+ ft going over Independence Pass. I use the 85 octane stuff and have never had a problem.

As far as gas mileage, I did recently install a K&N filter and the vehicle seems to get slightly better mileage, although I've never figured the mileage with pen and paper, but rather just gauged it by the computer. Don't know if the K&N had much of an effect other than replacing a dirty filter with 15K on it, but I've always used K&N filters in my vehicles and bikes.

Seem to get about 20 mpg combo of around town and highway (by the somewhat inaccurate computer). When going downvalley (losing 1000+ ft), you can cruise at 70-80 mph with a very light touch on the gas pedal, reset the computer, and get it up into the 40-50s mpg. Then I pretend I'm driving a Prius.

Yeah, the air's thin here (tell that to my lungs!) and I guess there's less air resistance at higher altitudes. Maybe that's why my V8 gets 400 MPG when I'm at the top of the pass (12,049 ft)! :lmao: