Say bye to hypermiling at average 65+ mph?

Discussion in 'Gen 3 Prius Fuel Economy' started by vegeto626, Jan 17, 2012.

  1. kswebb1

    kswebb1 Junior Member

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    I have found that at 68 up to 73 the difference in fuel use is not noticeable.Now fallowing trucks for extended amounts of time at a safe distance does help with fuel mileage.But for a short time adds very little if any real world improvements.Why??Because you get more resistance when you pull out to pass the truck which reduces your mileage to the point that it evens out.In fact I go in the third lane to pass trucks so I don't hit the air they are pushing.

    The Biggest reliable improvement I have found is airing up the tire pressure.And even this has a small drawback.When you change tire pressure and run a lot of highway you actually drop mpg for the first 70 to 200 miles.I think this is from the different curve of the tire causing the tread to hit on the tread different until it wears a little.

    I drive 78 miles each way to work or 156 round trip with 95 percent of that highway.So if anyone has any advice other then slowing down.I would be happy to hear it.Speed limits 70 so I would cause and accident if I slowed down.
     
  2. F8L

    F8L Protecting Habitat & AG Lands

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    While I agree that close drafting is dangerous and not very wise, distance drafting can still significantly raise mpg. I have observed over the course of years that the Prius can still benefit from distance drafting yet maintaining a safe and comfortable following distance. Between my Scangauge and the MFD it is easy to quantify the gain.the cons regarding rock chips and other road debris still remains but is significantly reduced in distance drafting.

    I can easily observe a 10mpg+ gain by drafting even 20 car lengths back. Because I drive 95% or more freeway miles I have plenty of practice testing various drafting lengths. The fact that I can often follow trucks on the majority of my 46mile (one way) commute can make the difference between 57-60mpg tank and a 50-55mpgntank.

    That being said, the difference in money saved is rather small as is the actual fuel usage so I never recommend this aspect of hypermiling but I will do it myself when I am trying to push a tank or just so I can drive at the speed limit and not feel like I am in everyone's way since they would have to go around the large truck anyway. I enjoy driving between 60mph and 65mph and following trucks is about the only way I can maintain this speed without being a road hazard. Anything higher than 65mph really starts to tank the mpg on a GenII.

    I agree with PriusGirl in spirit, however. Try to be perpetuate the slow and dangerous Prius stereotype which in my opinion is created because of slow old age drivers and less from hyper milers. In my area most Prii drivers are pretty old.
     
  3. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    Basically you can save more by simply slowing down than you can by drafting a truck. Why?
    - aero drag goes as the square of the speed, so simply slowing from 70mph to 60mph will reduce aero drag loss by 27%, which is about what you save by drafting a truck at the hyper-dangerous distance of 20 feet (at 55 mph, unfortunately we don't have the numbers for 70mph but you could probably be slightly further back)
    - but going at 60mph also reduces the energy losses due to friction (engine, trans, and bearings) and tire flex
    - and as noted you can drive more smoothly with less accel/decel than you can close drafting, which saves more gas.
    - if you're going to draft a truck, you have to go at their speed (of course you can pick a slower or faster truck)
    - of course, drafting further back at a "safe" distance will mean less drag reduction

    Today I drove 120 miles on I-95 and on suburban roads to/from it and averaged 63.4 mpg on the HSD (probably 60mpg actual), temp 35-40F, winter gas, 44psi stock tires. I-95 is zoned 65/55mph, I was at 50mph except for slowing for some 45 and 40mph construction zones. Does anybody out there get mpg like that from "safe" drafting a truck?
     
  4. F8L

    F8L Protecting Habitat & AG Lands

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    You're missing my point. Driving less than the speed limit (65-70mph) in my area is dangerous or at least inconsiderate at best. So driving 60mph is the minimum I am willing to travel unless the roads are empty. Under normal conditions the only way to travel slower is to use a large truck as a shield. Thus you are able to travel slower and draft at the same time. This is especially important for GenI and GenII owners who were not blessed with either the aerodynamics or larger engine of the GenIII. Traveling at 60mph in a GEnIII is a little like traveling at 50mph in a GenII according to the charts I've seen. Sooooo drafting a truck at a distance is sort of like driving a GenIII at the same speed but without a truck.

    To answer your question, yes I can average well over 60+mpg when drafting at a distance at around 55mph to 60mph. In summary I agree that traveling slower is the best option for reducing fuel use but it's not so easy to do on crowded roadways so using a truck as a shield is the next best option and in my testing the mpg is even higher. You do not need to be 20' or less from the back of the truck to benefit. I am careful in my observations and the MFD and Scangauge does not lie. :)
     
  5. SageBrush

    SageBrush Senior Member

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    I have tried F8L type drafting and gave up on it when I realized that the extra concentration required of me to add a bit of safety to the endeavor was a lot more tiring than simply leaving a few minutes earlier for the trip and driving 65 mph rather than 70 mph.
     
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  6. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    Everybody has their comfort zone. Six years ago when I started doing this I was very uncomfortable moving slower than the flow of traffic, so I slowed gradually and eventually found that a base speed of 50mph worked for me. The legal minimum around here is 40mph, but personally I wouldn't be comfortable going that slow. That said, I modify the base speed, going slightly faster when there is traffic behind me and slowing a bit when the road behind is clear. Giving semis a few flashes of the hazards when they are still far behind me helps them get by and keeps things smooth.

    Other factors:
    - traffic naturally slows down when it is heavy (often to my speed!)
    - when traffic is lighter, it clumps so it is easy for the clumps to get by and then there are gaps
    - often people are so preoccupied with their (illegal handheld) cell phones that they are perfectly happy to follow along behind me (today for example a woman followed me for miles, not passing even when the next lane to the left was clear)
    - I-95 west of New Haven is 55mph so I have zero qualms about 50mph there
    - construction zones are 40mph and 45mph, and that's what I'm doing there, safety of the workers trumps "inconsiderate" any day

    I haven't driven in CA lately. But I repeatedly see people saying that in CT, NY, NJ, MA they "have to" drive 70 or 75mph or get run over. That has not been my experience in my ramblings on all the major NY highways, the NJTP, all the highways in CT, the MA Pike, I-91, etc. So, sorry, I have to be a bit skeptical when people say that they are forced to speed in other areas.

    Since mpg depends so heavily on speed, when someone says they "have to" drive fast, that will largely determine their mpg, the other effects are smaller. But if they're not comfortable going slower, they shouldn't and should just accept the mpg they get at the speed they want to go. Same is true for me, I'd probably get better mpg at 40mph, but I'll accept what I get at 50mph.

    Mythbusters found 10% mpg increase at 100' at 55mph, so the minimum safe 2-sec following distance (150 feet) would yield something less than that, maybe around 5-8%, or 3-5mpg at 60mpg. Not insignificant.

    P.S. FWIW, today's mpg's were winter numbers. Summer figures would be roughly 65-70mpg actual for this trip. I have HSD photos for two trips to Boston, which give somewhat better mpg's than today's route, showing 72.5 and 76.8mpg (actual is about 5% less). So non-drafting 50mph does work nicely.
     
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  7. fuzzy1

    fuzzy1 Senior Member

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    I think F8L's point is that in his particular region, he can't go slower without inciting California-style go-faster-or-get-the-****-off-the road rage for other drivers. Without using the truck as a screen against other traffic, even the speed he is suggesting would likely provoke that rage.

    I have used that method too, up here, when I want to do just 60 when heavy traffic wants to go at least the posted car limit of 70 and has critical mass to force it. There are always a few trucks obeying the truck limit of 60 as a condition of employment. By hanging 3 or 4 seconds behind, I'm not sure I'm getting any slipstream help from them, but I do get it from the many passing vehicles. And I'm close enough that would-be ragers instead choose to move left and race me to the truck. I let them win every time.
     
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  8. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    That matches my observation that 'not drafting' gains me 3 MPG.
    I usually am following a vehicle (that just passed me) at whatever cruise control setting lets him pull away at the slowest difference possible. Mind you, depending how soon they cut in front of me, I may not have the following distance you (or I) want.
     
  9. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    If I wanted to use a 60mph truck as a shield from faster traffic coming up behind, I'd get in FRONT of it and travel at the same speed. Better protection, less debris, same slipstream from passing vehicles.
     
  10. fuzzy1

    fuzzy1 Senior Member

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    (1) If you stay far enough in front to give the truck safe following distance, plenty of vehicles passing it are going to pull in behind you before realizing you are also going slow. Some of them are bullies, displaying anger to cars that they won't display to trucks.

    (2) When I've used this (traffic on these routes usually isn't heavy enough to be that pushy), I've been able to sit far enough back to not have debris problems, and still get the screening benefit of the maniac speedballs focusing on the truck, not me. Crosswinds may have been been blowing the debris out of my path.
     
  11. NineScorpions

    NineScorpions Economy, Meet Style!!

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    I have a 26 hour one way trip headed my way...so I will try a little bit of everything since there is not much else to do but listen to the radio :D
     
  12. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    For those who claim they have to do the speed of light or photons are rude when they pass them by.

    [​IMG]

    I saw several signs and one wagon on US 65 in Missouri.
     
  13. jhinsc

    jhinsc Senior Member

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    While hypermiling may not be entirely possible, you can still get great mileage at highway speeds. Going from SC to NY and back over the holiday's netted me 52 mpg average going about 5-8 mph over the speed limit, mostly on I-95. I was still being passed by the crazies out there and saw two get speeding tickets - ha ha! I also used cruise control about 60% of the time. Inflated tires to 39/36 and lower grill blocked. Not bad at all!
     
  14. msirach

    msirach Member

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    Drafting is NOT condoned or promoted. DO NOT DRAFT.

    To follow most trucking company safety standards: If a truck is in front of you, back off until you can see the rear view mirror of the truck. This assures that the trucker can see you.

    You can maintain a 3 to 5 car length distance and reap more than 3mpg. The pace of the truck will keep you at a steady throttle pressure and that will increase your mileage.
    A Gen III is worth 65mpg at 65mph.
     
  15. Insight-I Owner

    Insight-I Owner 2006 Insight-I MT + 2011 Prius

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    Two-second rule - Wikipedia, the free encyclopedia
    Two seconds at 60mph is 88' x 2 = 176' = 12 Prius lengths (15'). More at 65 or 70mph. Less than that is drafting/tailgating, sorry, and may get you a ticket in some states if the cops are in the mood. 3-5 lengths = 45 to 75 feet, which is less than one second even at 60mph.

    From my experience, I estimate that the Gen III is good for 60mpg (actual, not HSD) at 60mph in the summer. If you want 65mpg, you'll have to be going slower than that.

    Sorry, I can't see how being visible in a trucker's mirror is safer for a following car. Simply moving to the left side of the same lane keeps me visible quite close behind the truck, so this doesn't guarantee a specific following distance. And if the trucker sees me, what difference does that make in an emergency situation? If there is danger ahead he'll still slam on his brakes. After all, he's got 83,000 lbs (more if he's running illegal), most of it behind him, so delaying his braking because I am following him can seriously endanger his life.
     
  16. Codyroo

    Codyroo Senior Member

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    One other point brought up to improve miles per gallon on the OP's new, longer commute. Try not to tailgate. Leave a 2 second gap (~180 - 200 feet) between you and the person in front of you. By having this gap, you'll be able to see, more easily, what traffic in front of you is doing.

    If traffic in front of you is slowing (you'll see brake lights from ALL the other drivers, because they are tailgating) you can reduce speed in this order (allowing the best MPG)

    1) Let off the throttle and glide to reduce your speed.
    2) Let off the throttle all the way and coast to reduce speed (this includes that little bit of engine braking you see on the HSI display. Once you have matched speed, return to #1.
    3) Brake only using regenerative braking and let off the brake to return to #2 or #1 if you've matched the speed of cars in front of you.
    4) Brake using regen and friction brakes until you are close to matching speed of the cars in front of you, then revert to #3/2/1

    In slow and go traffic, it is critical to leave a gap in front of you. This way you can pulse and glide while minimizing the use of brakes.

    It's the damndest thing, isn't it. If a car comes up on a truck (or bus) driving between 55 - 60 mph in the right lane, they pull to the left and pass it. If the same car comes up on another car driving 55 - 60 mph in the right lane, they feel they have the permission to honk, tailgate, pass & glare, flip off or perform some other act of anger display because their expectations weren't met.

    I respect Pinto Girls assessment, and agree that her assessment is accurate of the typical driver's opinion of a slower driver. However, I refuse to be pressured into driving faster and tailgating simply because that's what other drivers want me to do. Much in the same way I don't drive 65 mph in the #1 lane because I feel the need to be indignant about everyone driving the speed limit.
     
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  17. Fred T. Jane

    Fred T. Jane New Member

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    Last week we had some pretty high winds that was noticeably eating away at my mpg as I was driving home to the point I was getting only 40-45 mpg on the MFD. The next day with the same winds I got behind a truck and stayed about 3 car lengths behind him. The MFD's mpg display showed well over 50 mpg, and as the big fellow sped up to about 70, I kept pace with him and the MFD was showing 60 mpg the entire time. So yes, even a modest distance behind a semi will still get you some benefits of drafting.
     
  18. fuzzy1

    fuzzy1 Senior Member

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    Just be aware that a mere 3 car lengths at 70 mph is serious tailgating. At the speed, the common Two Second Rule corresponds to nearly 14 carlengths. Legal minimum in my state is more than 9 Prius lengths, not that it is enforced or many drivers know it.
     
  19. ItsNotAboutTheMoney

    ItsNotAboutTheMoney EditProfOptInfoCustomUser Title

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    Situations requiring heavy braking don't necessarily require you to slam the brakes immediately. In fact, even in an emergency stop you're taught to apply increasing pressure to help reduce the risk of a skid. The initial light braking is enough to light your brake lights and trigger a reaction from the driver behind you (and the driver behind them etc) and can be the difference between a pile-up and sighs of relief.

    Given that a truck brakes more slowly than a car, it's the reaction period that is the key to avoiding a collision. If the truck driver can see you, they can see how far you are behind them and as long as they're checking their mirrors they aren't going to forget that you're there. That at least gives them a chance to make an adjustment for your presence.
     
  20. Fred T. Jane

    Fred T. Jane New Member

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    Around where I live, three car lengths is giving someone a nice buffer--no matter their size. In my defence, I made sure I could see his rear-view mirrors and he could see me. So thinking about it, I might not have been as close as three car lengths.

    Maybe when Google takes over the world and we all drive self-driving cars this discussion will be moot. :)
     
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