Prius Milage Doesn't Like Texas

My guess is that you're running the A/C constantly in Texas and a lot less in New Mexico.

 

Higher altitude, less air resistance is a big part of your difference.
If my arithmetic is correct, air is about 25% lighter in NM than Texas. That should be worth around 12% difference in fuel economy if humidity and temperature are similar.

 

Despite old claims that engines are less efficient at higher altitude, I've seen dozens of posts over the years with people stating they get higher mpg at 5-6,500 ft then they do at sea-level.

The only detail I can think of is the fact that in thinner air it requires less accelerator pedal to travel, and with the ocean in sight you have to mash the pedal just a little harder to get and stay moving. Just a theory, I'm sure somebody with some real knowledge will step in eventually.

If you come from a place will hills which you glide or coast down, and the area you traverse in Texas is mostly flat, that too could create a significant mileage loss.

I recall driving a cross open desert in Texas and not going anywhere near the speed limit just to save time. Nobody does 40-50 mph on the open roads down there. The difference in mpg between 55mph and 75 mph is noticeable.

 

The altitude efficiency part always bugged me because while the air is thinner causing less resistance, you make more horsepower at lower altitudes because of increased air pressure. Texas race tracks have always been favored because of low altitude air density opportunities presented by geography and weather (obviously not in summer). Dense air is great for fast 1/4 mile racing times!

How were the roads in Texas? Road surface changes can alter fuel efficiency quite a bit. Cobble-like asphalt is terrible compared to smooth finished concrete.

 

AC probably has a lot to do with it. I usually drive with AC set to 80*F in ECO mode, and get around 50MPG on the MFD. Last night the heatwave moved out and I was able to drive with AC off and windows opened a little. I was getting 60MPG readings for the similar trips!

 

There could also be effects from traffic. For example, if there are differences in how many cars are on the road, or how quickly drivers in the area tend to accelerate, or how the lights are (or aren't) timed, or where the sensors are for traffic lights.

For example, in my area, they put four sensors all in a row right at the light, and almost never further back. If the light's red as you approach and nobody else has tripped the sensor, you're doomed - you have to stop. Where I used to live, they put one sensor at the intersection, and one set back a good distance; if you approach it slowly, you can often coast through at 20 mph or so. One brand new traffic light in that area even turns red in all 4 directions at night; the instant somebody passes over any sensor, the light turns green immediately in that direction, so you don't even have to slow down at all. That area also seems more enlightened when it comes to timing lights; when driving on the main streets in town, as long as you travel right at the speed limit, you won't get caught at any red lights.

Point is, there's lots of things that can make a difference; the other ideas above are all good too. Even just familiarity can make a difference; maybe you stop just as often in both places, but you know where you're going to stop, so you anticipate and start coasting earlier. Or maybe they use a different material for the road that is softer or rougher and increases your rolling resistance?

 

More HP does not prove better fuel economy. I'd also hesitate to draw conclusions from racing where the air throttle is presumably always wide open, compared to usual street driving where the opposite is mostly true.

If I understand correctly, cars are programmed to mix fuel and O2 in ratios that deviate neither to rich or lean for efficiency and emissions reasons. If the car takes into account the lower partial pressure of O2 at higher altitudes, *more* air will be allowed into the combustion chamber to maintain stoichiometry compared to sea level. This is through less restriction of the air throttle valve during partial power ICE use. In that case the car not only pushes through thinner air, it runs with less air restriction.

 

I'm one of those people. My last three non-hybrids all did better on high MT-WY-CO roads than on sea level WA-OR roads, as long as steep mountains descents didn't interfere. Shallow slopes don't hurt. (I'm comparing only the modern closed loop pollution control systems, typically fuel injected, not the old uncontrolled open loop carbureted systems.)

Many people confuse power with efficiency. These are not the same thing. Best efficiency and best power are often in conflict.

The thinner air means engines do have less power available at high elevation. But American cars have so much more power than needed for regular cruise speed that this is irrelevant on flat and lightly sloped roads.

But in non-hybrid Otto cycle engines, the low cruising power produces a lot of engine 'pumping' loss. This loss decreases significantly with the lower air pressure at higher altitude. The Atkinson cycle engines in hybrids get this improvement, and more, all the time regardless of elevation. So high elevation helps hybrids less than non-hybrids.

Reduced air drag is a very big factor, hybrid or not.

Note also that high elevations often have 85 octane gas, which (at least in the old days) tends to have a bit higher energy content. Again, don't confuse energy with power. Powerful high octane engines get their power from guzzling more fuel fast, not from using higher energy fuel.

The tires of my old nonhybrids always ran door placard pressure (or 32, on a car that easily hydroplaned at its recommended 26) at sea level. Remember that as the absolute pressure outside falls with altitude, the gauge pressure in the tires should rise by the same amount, absent temperature change. This was thus boosting the effective tire pressure, which we now know helps mpg.

Those are the factors I remember at the moment, there may be more.

 

High altitude is also why today all diesel passenger car and truck
are all turbocharged, so more forced air could be supplied to the engine.

 

Isn't it a way to up the effective compression ratio?

 

Every time I was back home in Texas I noticed all the pumps had at least 5% ethanol. Might have just been the DFW area but not sure. If that is the case wouldn't that effect your MPG?

*edit* Ok I check, almost all gas stations in Texas use around 10% ethanol.

 

That can definitely make an impact. Is this common with some states, they just use E10 in everything?

Here in Nebraska (where you think they would be pushing E10 even harder) we only have one grade at each pump that has E10. It is usually about 10 cents cheaper than E0 so I usually pay the little extra for the E0. I figure it's a wash because of the better FE I'm getting not using E10.

 

We don't have a choice in Cali. It's all E10.

 

Ya, the more I think about it, the stranger it is. We are one of the nation's largest producers of ethanol/corn and E10 isn't even forced on us like it is in a lot of states.

 

Also, that makes your MPG numbers even more impressive since you aren't getting as much energy from your gas as I am.