Does the gas in Bakersfield suck?

[PriuStorm]

Hello folks...

Twice have I taken the trip from Davis down to Bakersfield and twice have I noticed this drastic difference in fuel efficiency.

This last time over Thanksgiving, we drove down... I drove primarily both ways, and had the cruise control set on 68mph on the way down. We reached an average of almost 50mpg (49.8) on the way down.

On the way home, we fueled up in Bakersfield before taking off. Again I drove, and this time averaged a speed of 70mph on the way home. Fuel efficiency was so poor right off the get-go that I decided to 'help' when we got to the hills... took it off cruise control, allowed the car to gently slow to ~62mph as we were climbing the hills, and when going downhill, I'd increase back up to 70 again and resume cruise control.

Traffic was about the same in both directions, but on the way home we averaged less than 42mpg. That's an 8mpg difference. If I were driving my old Mitsubishi Mirage, or other 4 cylinder gas sipper, I would not expect that much difference just due to 2mph. Load was the same (except the two people may have been a combined 3 pounds more heavy (on the high side)).

Elevation is roughly the same, start to finish, so hills should cancel out. Is it the gas? Is there a tailwind going south down I-5 all the time? This is the 2nd time I've noticed a big disparity like this.

[JimboK]

Weather conditions, especially wind (as you've suggested) can make a big difference. A 5 MPH head or cross wind can reduce fuel mileage by about 8% or more, depending on the exact direction. So the difference between that small of a breeze as a head wind vs. a tail wind can be 16%.

[auricchio]

Yes, the prevailing wind will generally favor a southbound trip and penalize a northbound one. It can make quite a difference.

Also, I'm not convinced that hills cancel out if you cross them in both directions. A hill can be steeper on one side than the other, which can influence the car's performance. Technically, I know that it takes X horsepower to "lift" the car up the hill, and this should be constant no matter what the slope. But in practice this can be different, because the car is going to adjust the engine as necessary for the climb.

[topkick]

I don't know about the wind, but I drove from Oklahoma City to Houston on the 17th and got 50mpg for the trip down. Filled up in Katy coming back and had to fill up again in Denton, Tx today coming back to OKC. The trip back I got 39.7 mpg running about 70mph both ways. The big difference is that the fuel I got in Katy and Denton was 10% ethonol. Now I have a half tank to burn up and get some good gas again, hope I can find it.

[qbee42]

<div class='quotetop'>QUOTE(Rick Auricchio @ Nov 26 2007, 10:21 PM) [snapback]544525[/snapback]</div>

Also, I'm not convinced that hills cancel out if you cross them in both directions. A hill can be steeper on one side than the other, which can influence the car's performance. Technically, I know that it takes X horsepower to "lift" the car up the hill, and this should be constant no matter what the slope. But in practice this can be different, because the car is going to adjust the engine as necessary for the climb.
[/b]

Rick, your assessment about hills is correct. From an ideal standpoint, physics shows that the total work required to go up and down the same hill is zero, because the energy needed to go up the hill is recovered coming back down. In the practical world we have non-linear losses such as wind drag, and a limited ability to capture and reuse energy, which all add up to differences.

Tom

[Inches]

There are usually winds out of the north thus giving you a tail wind and therefore when you are heading home there will be a head wind and that would make a difference. Besides when going home you are heading up hill

[auricchio]

<div class='quotetop'>QUOTE(topkick @ Nov 26 2007, 08:26 PM) [snapback]544549[/snapback]</div>

I don't know about the wind, but I drove from Oklahoma City to Houston ...the fuel I got in Katy and Denton was 10% ethonol.
[/b]

Yes, prevailing westerly winds will favor the trip toward Houston, and penalize you on the return.

Ethanol has 20% less energy than gasoline, so that 10% in your tank got 20% lower MPG. That's about a 2% lower MPG reading, which is about 1MPG. Then you add in a factor for wind.

OKC is about 1300ft above sea level; Houston is essentially at sea level. So it was downhill from OKC and uphill going home.

qBee, I even forgot that rolling down hill isn't "free," because of wind and mechanical drag. And regeneration isn't 100% efficient anyway.

[hobba]

I had my 05 Prius in for service last week and noted to the service manager my recent loss of mileage. He told me they are getting a lot of complaints from customers these days regarding their gas milage. Toyota blames it on the gas blends being produced these days by the oil companies.