Highway MPGs Using DRCC: 63.4 at 60+ mph

Discussion in 'Gen 3 Prius Fuel Economy' started by a priori, Sep 25, 2009.

  1. a priori

    a priori Canonus Curiosus

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    People have asked: What kind of highway MPGs do you really get? Followed by: Really? At what speed -- 45 mph?

    This afternoon I drove from my office outside of Chicago to suburban St. Paul. The first couple of hours I wound through countryside and made a couple of stops in southern Wisconsin. I then got onto I-94 east of Madison and drove Interstate all of the way to St. Paul.

    I wanted to see if I could drive at highway speeds, using the dynamic radar cruise control (DRCC) on my Prius V w/ ATP, and get some decent gas mileage. By the way, highway speed to me means the speed limit (or slightly above, if going downhill!).

    My theory was that I could get very good results by shadowing (not drafting) 18-wheelers using the DRCC. By imitating the big rigs (gain speed on downhills and bleed it off on uphills), I thought I'd do better than setting the CC at a constant speed and then weave in and out of traffic.

    Today, the truckers really cooperated. I shadowed only two different trucks for the last 200 miles (199.7), which is also the period I measured. This is my measurement period because I was driving the entire time on the interstate using the DRCC, and I was able to write down the data starting at 6:24pm and ending at 5:54pm. This gave me roughly 200 miles in 3.5 hours -- not bad, considering I made one poddy stop and had a couple of construction slow downs. (The number turns out to be a bit better than 57 mph -- and that is just going by the clock, even when I was stopped with the car turned off and going through construction slow downs.)

    I set my speed at 69mph (speed limit was 65, except in two construction zones where it dipped to 55). Most often I was going slower than 69, because I kept behind (not drafting distance, but far enough to have cars get between us more than once!) a truck that would vary its speed. According to the MID, my average speed during this time was 58mph. Remember this also includes the time to enter and exit the freeway, come to a stop and start up again, as well as go through the two 55 mph construction zones. I'd say I was most often at about 65 while actually driving.

    What kind of mileage (fuel economy) did I get?

    According to the MID, I got 63.4 MPG. I could say this was at 57 or 58 mph, but the reality of it is that it was at higher speeds. Even though the majority of my time was at 65 mph, I'd accept that my real average for the time I was driving at highway speed was between 60 and 65. To accomplish this across a distance of 390 miles seems pretty impressive to me.

    I wonder what difference it would have made had I 15" wheels with 195 tires instead of the 17" wheels with 215 low profile tires (and only 7500 miles on them!)?

    Other factors of note: The sky was hazy and a bit overcast. Temps were in the mid-70s, falling to the high 60s by the end of my drive. The land was mainly flat, though the further north I drove into and through central Wisconsin, the hillier the terrain.

    "Real" MPGs: Now that I've measured 16+ tanks of gas, I feel pretty comfortable with my estimation that my car's MID reads 5% higher than the numbers I calculate. Applying this to the MID data from this trip (and assuming the odometer miles are faily true), it drops the very impressive 63.4 MPG to 60.4. That's enough for me to say that I can still get great gas mileage while driving on the highway:

    Even though the car reported my aveage speed at 58 mph, I am willing to say that my actual driving time was less, therefore my speed greater. I estimate my real driving speed to be in excess of 60 mph and as high as 65 mph.

    What does it mean? I am now very comfortable telling people the DRCC is more than just a cool toy. By mimicing a truck driver's actions, I was able to get better that 60MPGs. It really means that fuel effiency may be right around the Corner!
     
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  2. ALS

    ALS Active Member

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    It would only have made about 1-2 mpg difference.
     
  3. tumbleweed

    tumbleweed Senior Member

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    What were your tire pressures? Did you use the AC? If you did use AC were you in the ECO Mode? Did you use 3 bars the whole distance? I find on the freeway I sometimes need to drop down to 2 or even 1 depending on traffic conditions.

    I checked my mileage from here to Portland on I 84 using DRCC, about 200 miles each way. I had my DRCC set at 65 to 70 except the 20 miles or so when I'm near Portland then I drop it to 60. Last trip was about a week ago, AC on most of the trip, ECO mode, tires at 48 lbs. There are quite a few hills, no mountains. To many hills to be very sucessful following trucks as you were. The wind was blowing 30 Knots or so and I only got 46 MPG (displayed) going West. Coming back I over the same route I had a tail wind and managed 53 MPG displayed. Previous tests indicate my displayed mileage is about 2 to 2 1/2 MPG better than actual MPG. I think my Gen2 would have gotten about the same mileage.

    So maybe your trick with the trucks works pretty well, your mileage is certainly better than mine. But I don't think it will work around here because the elevation changes slow the trucks enough to hurt mileage.

    I'll be going over the same route again next month and I'll try to find some cooperative trucks and see what happens, maybe the wind won't be blowing and I can get some better numbers.
     
  4. royrose

    royrose Active Member

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    Are your figures for round trip or one way? Definitely useful and interesting info but one way never tells the whole story.

    Tumbleweed: I've driven that route. Is the wind ever not blowing?
     
  5. hobbit

    hobbit Senior Member

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    For starters, doing the bleed-off-on-uphills and get it back on
    the downside *is* an efficient way to drive, and every trucker
    knows this [whether they actually practice it or not, subject to
    their engine power]. But more importantly, even if you had DRCC
    on the "long" setting which the manual says is about 160 feet, you
    were less than 2 seconds behind the truck at 88 feet per second,
    and therefore drafting trucks at a distance that the truck drivers
    *really* don't want you to be at. My outside voice at this point
    wants to say "knock it off", but for the most part you're only
    endangering yourself in that situation. And setting a bad example
    to those around you.
    .
    Try this next time: mimic what the truck *would* be doing over
    the same terrain, by locking your foot at a particular HSI demand
    and letting things play themselves out but keeping an actual
    *safe* following distance from other traffic. Your results
    will likely be similar, but I'd be interested in which way
    the long-term delta is -- on one hand you maybe get a bit of
    aerodynamic help by the tailgating trick, but on the other you
    don't have to react instantly to any change made by the vehicle
    ahead and can thus ride right in your own sweet spot more
    effectively by leaving a nice big open "cell" to play in.
    .
    Sorry, DRCC and LKA are *not* substitutes for 100% aware driving.
    Not even close. Maybe when a tire cap comes in through your
    windshield, you'll understand.
    .
    _H*
     
  6. tumbleweed

    tumbleweed Senior Member

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    Hobbit, I share your concern about adequate following distances but the owner's manual is a bit misleading on that point. The 3 bars = 160 foot distance is only at 50 MPH. The bars are more closely related to time, 1 bar = 1 second, 2 bars = 2 seconds, and 3 bars = 3 seconds. It's not exact but it seems very close, I've checked it a number of times at speeds from about 40 up to 80 MPH. 3 bars equal to 3 seconds also means that the 160 feet at 50 MPH is wrong, it should be 220 feet. Perhaps others who use this system could confirm my observations, please? I would speculate that the software that controls the system may have been changed after the manual was written.

    The DRCC also seems to pay very close attention to the speed differential. If the car in front slows rapidly or if you approach a slower vehicle rapidly it will brake rather sharply to compensate and maintain a proper distance. On the other hand if the car in front is accelerating away from you, such as when someone passes and cuts in front, the DRCC may not slow you down at all, depending on how close it is, and just allow the distance to open up.

    But like any other vehicle control system it needs to be monitored because it could make a mistake. After 6500 miles, mostly on the freeway, mine has not gotten anything wrong yet but I watch it closely and I always will.
     
  7. nparker13

    nparker13 Member

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    Good to know about the DRCC. Unfortunately the highway I take to work doesnt allow trucks, and is very hilly, so I tend not to use it, but on long trips like the one I'm about to take tonight, I will definitely be more inclined (pun intended) to use it even uphill.
     
  8. tumbleweed

    tumbleweed Senior Member

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    Most of the time it is Roy, especially in the spring. Good place for wind turbines though.
     
  9. Celtic Blue

    Celtic Blue New Member

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    Let's be realistic. That distance is an eternity behind a truck or even a car on many roads. You will have cars weaving in between you and the truck at that distance. There is little for the trucker to worry about with you at that distance. The truck won't out brake you. And they don't seem to have trouble changing lanes right in front of you when you are passing... When you get locked in traffic there will be several cars between you and the truck at that distance.

    One should be more concerned about following sporty vehicles with short stopping distances. Whenever I have to brake hard I'm usually watching my rear view mirror to determine if I'm going to have to instead take evasive maneuvers to avoid being run over. There's nothing quite like the thrill of seeing an 18 wheeler barreling down a hill at you smoking his tires because some jackass stopped at the bottom of a hill in the highway. And while you ponder taking the ditch, having the car behind you beat you there...boxing you in so that he can avoid being run over. Leaves an impression even when the trucker does manage to shut it down a foot off your bumper.

    Where folks get into real trouble with truckers is side-by-side driving, especially in their blind spots. It gives them no room to maneuver and they might not be aware of you. The other problem is folks that pull right in front of them. The trucks need more room to stop.
     
  10. ken1784

    ken1784 SuperMID designer

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    Just FYI,
    The 160 feet distance is at slow speed, say 30 mph.
    The set distance is automatically increased as speeding up, say 250 feet or more at 60 mph.

    [email protected]
     
  11. a priori

    a priori Canonus Curiosus

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    I recall Toyota saying the difference was supposed to be about 4 mpg, but I don't know that there is any data out there. What is your basis for suggesting a 1-2 mpg difference?
     
  12. a priori

    a priori Canonus Curiosus

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    Thanks for the great response and comments. I hope you are able to find a way to use some of my experience to your benefit -- don't be worried about slow speeds up hills.

    Here are some responses to your basic fact-seeking questions:

    I measured the cold temp right before I left. The fronts were at 46 and the rears at 45. This was on the Toyo Proxes A20s (51 psi sidewall max).

    I used the A/C for only a short portion of the trip, because the sky was overcast and the temps in the low 70s.

    If you are asking about the traveling distance on the DRCC, then I had it set at 1 bar most of the time (2 for a little bit). In the heavy traffic areas, using 2 or 3 bars just wreaks havoc, because cars will be going in and out of this space all the time, and it causes too much braking and accelerating.
     
  13. a priori

    a priori Canonus Curiosus

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    Very Important Point: This is a one-way measure. My interest was to see whether use of DRCC for a long period to mimic truck driving would produce high MPG numbers. I'll try the same on my return this Sunday, though I may not be able to find two such helpful trucks driving at the same speeds I intend to follow.

    The wind ALWAYS seems to blow through central WI! Also, it seems to be blowing in my face -- no matter the direction I intend to drive!!
     
  14. a priori

    a priori Canonus Curiosus

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    I appreciate your concern for me, but the manual description doesn't appear to bear out the reality of the distance separation at the speeds I was driving. I will tell you that I had big rigs much closer to my tail than I was following the trucks. I also had more than one driver easily slip in between the truck and my car.

    Before setting the DRCC on this trip, I attempted to do much of what you just suggested. I was trying this on U.S. routes, state highways and country roads. I couldn't do it consistently, because the uphills took so much speed off that I would interfere with traffic too much.

    You've made quite a leap in assumption about my driving habits, hobbit. I don't deny you your opinions, but don't presume to know my driving habits.

    I drove the entire time with my eyes on the road, reviewing the rearview and sideview mirrors, and with my hands on the wheel. I used the LKA, at times, just to assure myself that if I did lose focus that I would receive a warning.

    As far as a tire cap coming through my windshield, this could happen at any time. The only time I could reduce this risk is to make certain other trucks would not pass me, and that I would stay a mile behind the one truck in front of me. True enough: If I stay farther back, there will be more reaction time. However, the farther back I am, the more cars and trucks there are that will fill the gap between us. These vehicles only block my view or add their own risks to that of the truck.
     
  15. Codyroo

    Codyroo Senior Member

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    Heck,

    I'm glad to hear that you've gotten your car back (again, and again, and...oh yeah...again). Hopefully the fourth time is a charm.
     
  16. ALS

    ALS Active Member

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    Wayne Gerdes (Xcel) over at CleanMPG was telling me that was what most people were seeing in real world driving.

    Think about it the tires are only slightly wider (two sizes) than stock. Has the aero number changed? , weight changed? Nope, only the road friction from the slightly larger tire contact patch. I'd bet the weight difference between the 17's and the 15's isn't as much as most people think. Remember the 15's also have hubcaps added on for a little better aero numbers.

    Now on the Prius the change also effects the rpms by increasing them on the V. The 15's are 832.6 rotations per mile verses with the 17's 845.1 rotations per mile. That is 1.5% difference to the bad.
    So if your running 1500 rpms with the 15's the V with the 17's would be running 1,522.5 rpms at the exact same speed. Not much but it would be burning slightly more gas. Add that in with the increased road friction and there is your 1-2 mpg.

    I think Toyota said 4 mpg to cover their asses. Remember people who buy the V's are probably in general driving them a little more spiritedly than the average Prius owner. :D
     
  17. brad_rules_man

    brad_rules_man Hybrid electric revolutionizer

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    wow! That is not fair! I have fought like crazy to achieve 55mpg on the MID, then it just takes getting in a hurry once to drop that to 50mpg. :( LOL I need to check my tire pressure maybe.. haha. I drove my parent's Prius yesterday and today and I think my dad had the pressure maxed on the tires. lol.. rode kind of rough. He has been getting 55 consistently in his gen2.
     
  18. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    have to say that Hobbit makes valid points about safety, but i am at a complete loss as to where we could exercise this. where i live, i cant maintain a following distance of 150 feet even if i wanted to (and i do!!).

    i can at times and the busier the traffic the greater the following distance i want. but on a busy freeway, i have a constant stream of people cutting in front of me. i ignore them and continue on since its normal for me to vary speed from 50 to 65 mph depending on traffic to keep from having to hit my brakes
     
  19. a priori

    a priori Canonus Curiosus

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    :wave: Thank you!!

    I'm just hoping to park far enough away from all of those "bad" cars.
     
  20. a priori

    a priori Canonus Curiosus

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    Thank you for providing such a full response.

    I think another reason for an expected MPG hit is the wheel/tire combo weight. Not only are the 17" wheels heavier, but the tires are also heavier. This additional unsprung weight supposedly has a pretty significant difference on fuel economy.
     
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