Highway MPG - "sweet spot" for steady level driving

[tacomel]

This past weekend I had occasion to make a round-trip from Madison to Chicago (Midway Airport) which for me is 95% highway driving. Without passengers, I could take most of the trip with vent on instead of A/C or windows down. And with little traffic, I could try some things out to try to maximize MPG. Overall I was pleased at the 56.0 MPG average reported by the MFD, with 387 miles on the tank, by my 2006 Prius (now with a total of 2100 miles on it).

I found myself on the consumption screen the most, catching the "current MPG" bar out of the corner of my eye. I found that between 63-67 MPH and on level roadways (which most of this trip through northern IL is) there was a "sweet spot" right around 75 MPG. I got there by accelerating to 69 MPH, easing up slightly on the accelerator, and then keeping a steady foot when the MPG graph got around 75. I was able to achieve this for several minutes at a time on some occasions, at a near-constant speed. (The cruise control seemed unable to achieve this state upon several attempts.)

At one point I got curious exactly what the car was doing so I switched back to the screen with the arrows. I had expected to see orange arrows to the wheels and a yellow arrow to the battery. I was surprised to find, however, that it spent at least half of the time with orange arrow to the wheels, a yellow arrow from the battery to the electric motor, and yellow arrow from the electric motor to the wheels. This is the same pattern I see when going up a steep hill. On many occasions, it flipped between these two states as often as twice a second, with the instantaneous MPG reading changing by about +/- 2 MPG (apparently imperceptible on the consumption graph).

My best explanation: The battery was at 6 bars and my best explanation is that it was at the "top" of 6 bars, almost ready to go into the green, but the computer wouldn't let it do that so it withdrew some power to the drive train. A second later, the battery was not "topped off" and it withdrew some power from the momentum to charge the battery. Repeat.

I'm posting because I have no idea if my explanation is true, and also to ask if there is an even more efficient manner -- besides simply slowing down -- in which the car can be driven at highway speeds (65 +/- 2 MPH).

[efusco]

More than likely you were in a 'deadband' condition. The arrows you were seeing were very minor swings in energy flow to/from the battery that were of very little significance.

[molgrips]

I've found the same thing. I think it works like this: If the driver summons more torque, current flows from the battery to supply it. If the driver backs off, energy goes from the ICE to the battery via the generators. In a steady state ie constant speed flat road, the car takes less and less charge from the generators until at a certain point it reaches equilibrium. This is shown on the MFD as flip-flopping yellow arrows.

[Billy]

Yep!, I noticed the same thing, if i stayed within the 55mph speed limit my mpg was not as good as if I were going about 65mph. So for the sake of the envoirnment I go 65 , oh, and I'm cheap too! :P I was also wondering if the name "Taco Mel" had anything to do with "The Annual Celebration Of Life And Rebirth Of The Earth Festival", If so, I am "The Slug",, hahaha,,, NO salt please!

Billy

[Rancid13]

<div class='quotetop'>QUOTE(Billy @ Sep 4 2006, 04:14 PM) [snapback]314206[/snapback]</div>

Yep!, I noticed the same thing, if i stayed within the 55mph speed limit my mpg was not as good as if I were going about 65mph. So for the sake of the envoirnment I go 65 , oh, and I'm cheap too! :P
[/b]

And you get there a little quicker too.

I've noticed the same thing too...the sweet spot is usually between 65-75mph; ideally, right around 71-72mph. I hate it when I'm in the sweet spot and holding my foot steady but lightly on the accellerator and I hit a bump in the road that jostles my foot and takes me right out of that sweet spot.

[barbaram]

I found it the other day about 60 MPH on a relatively flat section of rt 1! about 75 MPG for a driving interval!!!!

YEAH! unfortunately around here we have a lot of relatively small changes in elevation which will cut down on mileage.....

[Proco]

<div class='quotetop'>QUOTE(barbaram @ Sep 8 2006, 04:26 AM) [snapback]316222[/snapback]</div>

unfortunately around here we have a lot of relatively small changes in elevation which will cut down on mileage.....[/b]

This has been one of my biggest problems during my commute. I'm on the highway for 38-40 miles one way, but the combination of elevation changes and traffic make it really hard to maintain that sweet spot for long. Traffic is usually the killer on downhills & extended level road. There's segments of the Garden State Parkway that are custom made for "warp stealth" followed by extended sweet spot driving. But the volume of traffic during my commute usually prevents me from gaining any speed on the downhills, so any sweet spot segments get reduced.
<div class='quotetop'>QUOTE(Rancid13 @ Sep 5 2006, 06:21 PM) [snapback]314781[/snapback]</div>

I've noticed the same thing too...the sweet spot is usually between 65-75mph; ideally, right around 71-72mph. I hate it when I'm in the sweet spot and holding my foot steady but lightly on the accellerator and I hit a bump in the road that jostles my foot and takes me right out of that sweet spot.
[/b]

Oh I've had that happen, too, and it's a real PITA. Sometimes it happens to me when I'm stealthing home, too. I'll hit a pothole and the ICE will come on. Of course, other drivers are the bigger problem in NJ. We subscribe to the "Nature abhors a vacuum" school of following distances. If there's more than a car length between 2 cars, it must be filled as soon as possible. Never mind that the person following is currently getting 75mpg for a segment and you're screwing them up! [attachmentid=4921]

On the plus side, heavy traffic has been working for me the past couple days. I've got about 148 miles on my current tank and the MFD is telling me I'm at about 61mpg. The strange thing is I've got 2 pips gone on the gauge. I usually don't lose my first pip until near 150. Weird.

[barbaram]

I actually hit it doing about 60 the other day....since then practicing driving on the highway without cruise- but I agree, the agressive drivers here in NJ make it tough.

[V8Cobrakid]

i did the same thing the other day. i was on the road for almost an hour. I peaked at 73. I was traveling between 65mph and 72mph. If i went below 52, the effect wore off.

[curtissac]

<div class='quotetop'>QUOTE(barbaram @ Sep 8 2006, 03:26 AM) [snapback]316222[/snapback]</div>

I found it the other day about 60 MPH on a relatively flat section of rt 1! about 75 MPG for a driving interval!!!!

YEAH! unfortunately around here we have a lot of relatively small changes in elevation which will cut down on mileage.....
[/b]

I drive I-80 between Sacramento and the Bay Area at least twice a week and have tried several constant speeds with cruise to see what gives the best average MPG for the trip. 69 MPH. But as we all know, you mileage may vary.

During the late summer 69 MPH got me an average of 52-54 MPG. Speeding up to beyond 70 seemed put 49.xxx on the average indicator. Slowing to the low 60's doesn't seem to improve much.

The best mileage average mileage I have seen over a reasonable distance was ... now I can't remember... 62.x? Got that between the 76 at Red Top Rd. in Cordelia and Stanford. This was before I got my HOV lane stickers so I was in stop and go freeway traffic on I-80 and US101. Being stuck in other slow moving moving freeway traffic has shown that is when the car gets its best mileage.

As some others have indicated is normal, I have seen a slight decrease in my average mileage per tank since the weather cooled off. The car seemed to get better mileage when it was 90 - 100 degrees out. I am drving a lot in the early morning now when it's in the 30's and it seems to have a negative effect.