We on our road trip, and most of the motoring out of California has been at high altitude. Mainly above 4000ft. Despite going our usual speeds, we've set several record MPGs this trip, we got 34.8 blazing across I-80 in Nevada. The best we'd gotten previously was 33.7 which was a leisurely jaunt down Hwy 101.

So I'm wondering why is this doing so well? Does altitude help MPGs?

 

Technically speaking, assuming limits on the ECU's ability to adapt to pressure and oxygen concentration concentrations, higher altitudes provide less oxygen, which would richen your air/fuel ratio and use more fuel in the comparable time driven at lower elevations. The reverse would be true of lower elevations. The difference you saw was probably the result of open highway driving versus less open highway driving. My call.

 

But, to qualify the qualification that I made, if the ECU can effectively balance air/fuel ratio despite elevation, then elevation shouldn't make a difference.

 

The most comparable run to this one was blasting back up I-5, at about the same speeds. That got 30.1 MPG.

I did wonder about California fuel vs out of state fuel, but on the way out of California over the Sierras, we did 34.8 MPG as well. There's also the possibility of summer vs winter fuel, the previous bests were at new year. Or there's temperature.

 

All potential variables in accounting for it. Whatever it was, I say you need to take more road trips!!

 

Actually, you should get better economy at higher altitude. Since your oxygen sensor is reporting less oxygen, your ecu is going to lean out the mixture. This will reduce your power, but improve your economy.

 

My other thought was density of the air. Drag seems to directly related to air density, and at the altitudes we've been running at the air is 16-20% less dense than at sea level.

That should equate to significant MPG improvement, if half your losses was drag then MPG would increase 8-10% at those altitudes. That could account for a lot of the increase.

 

Another possibility, the tires are more pumped up at altitude. I measured the tire pressures today, I was surprised to find 37lbs in them. I put 33lbs in at sea level, and I'm now at 6700ft. I think that's more the ambient pressure is down 4lb, rather than the tires up.

More pressure should improve MPGs.

 

I moved from San Jose (near sea level) to Denver and noticed a few MPG increased in my daily driving. I don't know what to attribute it to but it seems to be true for me.

 

You'd get better mileage traveling from high altitude to low altitude
than traveling from low altitude to high altitude (downhill vs. uphill)!

 

Maybe, the air is thinner and there is less resistance for the car to move through? Any forced induction engine will be able to maintain full pressure in the combustion chamber, they may have to work a little harder but they can do it. That is why piston aircraft engines used turbochargers for years, to compress the thin air.

 

The best 3-tankfuls-average I ever got with my 2007 MCSm was 40 mpg -- and that was in the Colorado mountains, using Shell premium. I never did figure that out. With mostly highway driving here in relatively flat Iowa, I typically got 34 mpg with the MCSm.

I'm liking the thinner air theories. I wonder what Click and Clack, or one of their engineer buddies, would say.

 

The difference in air density between sea level and, say Denver, is about
15-18%. This could make a measurable change in mileage in terms of hp
needed to overcome drag at highway speeds. For the MINI, probably about
3-4 hp difference at 70 mph, which could change mpg about 5%. At lower
speeds, much less of a difference, as drag varies with the square of the velocity,
and the power needed to overcome air resistance varies with the cube of the velocity.

 

I'd vote for tire pressure increase at altitude being the cause.

 

I didn't keep track of my MPG change from IL to CO, but my JCW definitely has less power at higher altitudes. It's quite noticeable, even while driving around in Denver.

 

I'm surprised to hear that. In Colorado my MCSm was eager to climb and accelerate as it did. I had no objective measure of that, just an impression.

At the time I was recalling carburetor-equipped V-8s, years ago in the mountains, with pedal to the floor and losing ground

 

Yep, it was obvious to the old "butt dyno". No real measurement, just a feeling.

 

I live at 9,300 ft. in Colorado mountains in Summit County home to Keystone, Breckenridge and Copper Mnt. I drive to Winter Park Colorado almost everyday going over Loveland pass at 11,000 plus feet then onto Berthoud Pass also at 11,000 plus feet and I always avg. at the least 40 mpg. And thats while having plenty of fun in doing so. The car is a Mc 2007

 

Prescott is about 5,600 ft., but the hills surely negate any benefit described above.
Did you keep track of gas dilluted with ethanol? The corn syrup reduces the BTUs per gallon, so if any tanks were 100% gas, the mileage would be better.

 

There is an additional factor related to wind resistance that is at play. During level motoring the engine is always working to some degree against the aerodynamic drag -- especially with the MINI. The drag increases dramatically with speed, so much so that it is a major player at moderate and highway speeds. When motoring in the mountains, I believe the engine power necessary to overcome both drag and gravity while climbing is way more than made up for when you go through the pass and start heading downhill on the other side. In the mountains, whenever gravity is working nicely in your favor the throttle is closed and you're enjoying infinite fuel mileage!

I never was able to crack 39 mpg in my R50 for over three years of ownership. Last year, I screamed around the Rockies during MITM3 trying to keep up with all of the turbos and blowers in company. When I finally went to the pump, I expected the worst. Surprise! The calculation worked out to 42 mpg. Go figure.

 

It's worth pointing out two things.

1. The ECU cant lean out the mix too much in the high altitude because a rich mix is required to keep detonation from happening.

2. A turbo engine will perform different from a supercharged engine at alttitude. Remember you can adjust the amount of boost a turbo produces simply by fiddling with a waste gate where as supercharging is going to produce the same boost at a given RPM every time (assuming all variables are kept the same). So the reason a supercharge works at altitude is because you can have the waste-gate open later to account for the increased pressure differential between the boosted and unboosted side, a supercharger cant compensate like that (unless you oversize it to meet a specific manifold pressure at a given alt, which the MINI doesnt do). So if you ever get in a P-47 remember that you have to adjust the turbo as your altitude goes up and down, otherwise you will blow your engine or have no power.

 

A couple of remarks:

1. I don't know specifically about the MCS, but most car turbos these days use fixed wastgates. This means the turbo can boost a fixed amount above ambient pressure so as altitude goes up, the power will decline just like as with a non-turbo car. I don't think whether a car is turbo'ed or not will materially effect whether it gets better mileage at altitude.

2. The air density decrease at altitude (which does reduce aero drag) is a factor but unless you're in the really tall mountains, I doubt it is a huge factor, maybe a few percent at 5K feet or so. Everything else being equal, I'd bet a Mini might get a percent or two better mileage at 5K vs. sea level, perhaps a bit more if you were driving a very high speeds where aero drag really starts rising. But if you're up at 10K+ (very few roads are this high), the effect might be fairly pronounced, maybe 5% better mileage or so.

3. The ECU's in today's cars are extremely sophisticated and should keep the A/F ratio very close to a constant regardless of altitude. I don't think altitude would would change engine efficiency appreciably.

4. Mountains and elevation changes are likely to cause a mileage loss with driving technique playing a big role in the degree of loss. If you have to use brakes at all coming down, this is certainly going to hurt. Likewise if you go up the mountain at high speed such that the engine has to run at a much higher RPM where it is less efficient, this will hurt. I don't see any way that going up and down can help efficiency, at least not without doing some very odd driving techniques.

5. Mountains are colder than sea level, so if you're driving in the mountains where you're starting the car from cold, the warmup penalty might override all the other factors.

6. The tire pressure change is an interesting angle and generally higher tire pressures increase mileage. But the reduction on rolling drag effect of going from normal tire pressure at sea level to a psi or two higher at altitude probably is VERY small, less than a percent I would think.

7. All bets are off on mileage comparisons without doing it over a number of tanks. There just are too many variables (particularly how completely you fill the tank and the wind) to draw any conclusions on a single tank or two.

All in all, I think the mileage change in traveling from a sea level area to a high altitude area would be very difficult to separate from the noise.

Cheers,

- Mark

 

To revive my old thread, we went on another road trip this year, up the coast (all the way to Canada) so largely at sea level, with just a couple of day trips up to 5000ft. The trips were of very similar length (within 70 miles) and the mpgs were much the same (within 0.2). So maybe altitude wasn't the difference, long trips seem to have most influence on our mpgs.

Though for this years trip we had different tries (Continental DWS all seasons, vs Yokohama S.Drive summer tires), don't know how much of a difference that made.

 

Tires will make a difference. Air pressure in the tires will make a difference as well, and it will be higher if the tires are set at a low altitude and then checked at high altitude.

The air pressure helps in two ways. First, there is less drag at a given speed, which means less power is required to move the car through the air. Second, less-dense air is easier to draw past the throttle body than denser air. This is a smaller effect, but does make some difference.

The folks on ecomodder.com and cleanmpg.com know a lot about these things.

 

This would be my reasoning too!

Not to change the subject... 89AKurt, nice photo in your signature - very professional!