Model S

What is the most efficient travel speed?

edited November -1 in Model S
For us Europeans there is always a big question when we go on road trips and have to crosse Germany. Do we have fun on the Autobahn and drive fast, but charge more? Or is it less time consuming to drive slower and charge less? This week I took it to the test and compared driving 120 kph to 150-ish kph (wanted to do 160kph, but traffic was too heavy to allow for that)

Even though driving faster is consuming more, you don't need to charge that much longer due to lower SOC with high kW rate. Of course, the equation doesn't really add up anymore when you intend to drive 200kph for a longer time. At some point, consumption becomes too high and charging times too long.

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Comments

  • edited April 2018
    Depends on the battery, but the 85 peaked at about 78 MPH.
  • edited April 2018
    I have heard that the best recorded fuel mileage (in a test situation) was at a constant ~45 mph. This apparently is just below the speed where aerodynamic drag becomes relatively more significant.
  • edited April 2018
    This would not be much fun on the Autobahn, however!
  • edited April 2018
    Optimal driving speed to minimize total trip time depends on several things, and is not always the same. Supercharger spacing, distance off route, and battery size are some.

    For example, if you are starting with 100% charge, and your first supercharger stop is the same whether you are going 120kph or 150kph, then 150kph is optimal. But if you will just reach the first supercharger at 120kph, and would have to make another supercharger stop if you were going faster, then 120kph would be optimal.
  • edited April 2018
    @tes-s: that is not entirely correct. You are right in the case where you only need 1 stop to get somewhere. However, when you're road tripping and need 3-4 stops per day, then driving faster, leaving with less SOC than 90% is faster to reach your destination. I've been doing it that way for a few years now and each time the results are the same.

    @Bighorn: it may depend on older batteries, but I get the same results for my earlier P85D and now the P100D. Maybe the software limited 60Ds might even get a bigger advantage of being able to charge longer at higher rates?

    @docdac: 40kph or 25mph is the optimal speed for the lowest consumption. Tested and used that in my hypermiling record setting run :-). But that would be illegal on highways and certainly not fun for the family to travel this way. I'm talking about efficiency in total driving times, not in getting the most miles out of a single charge.
  • edited April 2018
    "However, when you're road tripping and need 3-4 stops per day, then driving faster, leaving with less SOC than 90% is faster to reach your destination."

    That is not entirely correct. If you could charge to 95% and avoid a big detour, that could be faster. For example driving in a 60 from Baltimore to Buffalo. Wouldn't a big charge in Harrisburg be the fastest option?

    That is why I said "it depends". There are some general rules that are helpful, but no absolute rules on speed or charge level.
  • edited November -1
    I'm surprised this is coming up again. I thought this was settled a long time ago, that the quickest distance between two points, including charging time, is to drive as fast as possible with the stipulation that you need a big enough battery to get to your next supercharger stop. I understand that this may be oversimplification, but without any other factors in play, drive as fast as you can because charging speed is always faster than driving speed, unless you're stupid and charge to 100% at every supercharger.
  • edited April 2018
    Depending on what you mean by “efficient”, the hypermiling record for a 100% charged Model S P100D is 670 miles, achieved by maintaining an average speee of 23 mph.
  • edited April 2018
    He means travel efficiency not distance records.

    @finished
    Ironically, it is you who are incorrect. It’s been mathematically modeled based on the taper. Your confusion arises over equating statutory miles with rates miles.
  • edited April 2018
    For me, about 65mph seems to be the sweet spot for my 2015 P90DL. Unless you have an Autobahn, going much faster than that makes little difference because of time it takes to exit, drive to the SC, and spend 5 to 30min charging. Then there's no guarantee the SC is available or charging at max power. Also you have to be willing to attract the attention of law enforcement, and an extra 15 to 20mph isn't worth the risk to save mere minutes. I'm getting about 93% lifetime efficiency which is almost exactly what Tesla says you get with 21" tires.
  • edited April 2018
    Unless I'm completely off with my math, as long as the charging speed is faster than the driving speed, you're always better off going faster. e.g if your charging speed is 200 miles per hour of charge, as long as you're driving under 200 MPH you'll save time driving faster.
  • edited November -1
    > 200; 78, 65, 45, 23 mph-- and 200 or 150 kph?
    There must be a single answer.
    I vote with DocDac.
    45 miles per hour-- that's when the air speed begins to provide significant drag.
    I remember when I was 12, that the neighbor drove from L.A. area to Alaska at 55 mph one year and then 45 mph the next year, and had better gas mileage.
    I know we don't burn gas in our Teslas, but same point, I would think.
    I remember 1974 when the USMC at Camp Pendleton decreed that we drive 45 mph to conserve fuel (anyone remember that?) and LtCol O.K. Steele would drive exactly 45 mph in his old Volvo and there would be a line of us behind him every morning, no one wanting to pass.
  • edited November -1
    Here's a good short piece: https://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question4771.htm

    Tesla Model S and 3 have extremely low drag coefficients-- can we drive our Teslas faster and have the same efficiency as our neighbors in their Ford F 150s?
  • edited April 2018
    @Chunky
    You are also conflating rated miles with the energy consumed to drive an actual mile. If you convert to watt hours, you’d see how driving very fast consumes the equivalent of 2 to 3 rated miles per mile covered. You may add an average of 200 rated miles an hour but it only gets you 66-100 miles of actual driving capacity. Efficiency peaks between 78-82 mph, after which efficiency degrades. As mentioned above, the distance between chargers will impact this, but it’s based on average distance between chargers, I’m remembering 120 miles, perhaps. If you’re going to skip a charger, the most efficient speed would drop as you’d enter the slow upper 20% of the battery to achieve that.
  • edited November -1
    Also interested in the most efficient speed for Model S versus Model S.

    Different coefficients of drag, therefore, different most efficient speeds?
  • edited April 2018
    Bighorn, you should read the OP post more carefully and maybe watch his video. He's not asking what speed is most efficient in terms of Wh/mi, but rather overall trip efficiency in terms of getting to a final destination in the shortest amount of time, including charging. Driving faster (as long as you have sufficient battery to do so) = shorter overall time. Charging speed is always faster than driving speed up to high SOC levels. Efficiency (wh/mi) is irrelevant if your battery is big enough to allow you to drive fast to get there. That is the question he's trying to get an answer for.
  • edited April 2018
    @vp09 you mean Model S versus 3 or X? For sure the most efficient speeds will be different, but not by much I suppose...

    For Model S, I like to use this table for planning my trips between chargers and highly recommend it: http://dauger.com/tesla/supercharger.html. Just remember to convert miles to km if you drive in Europe.

    As to the original question, I work as an editor at https://sportsperformancetech.com meaning I get to drive A LOT between different sports fairs and shows across the US and note down every trip details like mileage, avg speed and charge times (yeah, I am bit of a numbers person...). So based on my data, I get the most bang-for-the-charge at below 50mph , fairly in line with what others say.
  • edited April 2018
    @Bighorn : you are right that I was mixing and matching units. But the math still seems like it would work - as long as you are driving at a speed lower than the charging rate, you're fine. If you're charging at N rated miles per hour, as long as your speed is below N rated miles per hour, you're OK. For a charging speed of 200 rated miles per hour, I don't know how many actual miles per hour that is. I can't imagine on a US road you'd get 50% of rated miles driving on a fairly flat highway. When I drive about 70, I am at about 85% efficiency or 82 rated miles per hour.
  • edited April 2018
    For what its worth, when using the adaptive cruise control and going to Williamsburg on 95 and 64 I usually end up with a 30 mile average of 250-255 whr/mi. Based on traffic conditions and my setting of 10mph over the limit I’m usually doing 65-75 mph.
  • edited April 2018
    @finished
    I’m well aware of the OP’s intent and have been explaining this concept for 4 years now. You’re not understanding what I’ve written.

    @chunky
    In order to achieve rated miles at highway speed, you need to average around 20 kW. In hilly country or with headwinds it’s not difficult to see power draws of 40 to 80 kW especially when approaching triple digits. What people fail to realize is that a 200 mph charge rate does not correspond to driving 200 mph. This debate took place in earnest several years ago. Driving faster than 80 begins diminishing returns because of drag primarily. The upshot is that it’s only possible to average about 50 MPH in an 85 and 55 in a 100 over long distances. Driving 120 won’t change that.
  • edited November -1
    @Bighorn : I remember those discussions and I was not moved by the arguments. I don't recall seeing any math to back it up, but rather more of a "it makes sense" kind of arguments which don't sway me. I could be wrong.
  • edited April 2018
    What would sway me is seeing Wh/Mi for various speeds. Does such a chart exist? I can't imagine, under normal circumstances, anyone being able to drain the battery faster than a supercharger could fill it up
  • edited April 2018
    The worst Wh/mi I have ever had, on a flat road, was just under 400 going about 75 with a strong headwind. So that would be about 276/400, or 69%, which would correspond to about 108 rated miles per hour.
  • edited April 2018
    @chunky
    There was a nice white paper on one of the EV sites with graphs. People are impressed with the high initial charge rates when it’s 300+ MPH, but when the kWs drop to 50 or 30 or 10 depending on how full the battery is you may only be adding 30 MPH at that point. It’s blunted by the fact the the MPH figure is a session average. Also keep in mind that you burn a rated mile in 278 Whrs but it takes 300 Whrs to replenish it. Plenty of lead footed P owners report lifetime average efficiencies over 400 Whr/m, so it isn’t too hard to imagine approaching 556 on a cannon ball run burning two rated miles for every mile covered. Maintaining that inefficiency would require one to near range charge every 120 miles which woukd take close to 90 minutes. After close to 1500 supercharger visits I guess I’d just say you’re going to have to trust me that the math matches reality.
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