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## What is the most efficient travel speed?

## What is the most efficient travel speed?

Submitted by steven on April 15, 2018

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.

Bighorn| April 15, 2018Depends on the battery, but the 85 peaked at about 78 MPH.

docdac| April 15, 2018I 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.

docdac| April 15, 2018This would not be much fun on the Autobahn, however!

tes-s| April 15, 2018Optimal 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.

steven| April 15, 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.

tes-s| April 15, 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.

finished| April 15, 2018I'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.

Mi75d| April 15, 2018Depending 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.

Bighorn| April 15, 2018He 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.

Darthamerica| April 15, 2018For 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.

Chunky Jr.| April 15, 2018Unless 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.

vp09| April 15, 2018> 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.

vp09| April 15, 2018Here's a good short piece: https://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question4771...

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?

Bighorn| April 15, 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.

vp09| April 15, 2018Also interested in the most efficient speed for Model S versus Model S.

Different coefficients of drag, therefore, different most efficient speeds?

finished| April 16, 2018Bighorn, 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.

dianalucas| April 16, 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.

Chunky Jr.| April 16, 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.

jamenendez01| April 16, 2018For 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.

Bighorn| April 16, 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.

Chunky Jr.| April 16, 2018@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.

Chunky Jr.| April 16, 2018What 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

Chunky Jr.| April 16, 2018The 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.

Bighorn| April 16, 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.

Bighorn| April 16, 2018And here’s the math:

https://teslaowner.wordpress.com/2014/12/09/driving-charging-time/

Chunky Jr.| April 16, 2018I'm sold! Thanks for the data!

Bighorn| April 16, 2018Excellent! Maybe you can explain it to @finished:)

tes-s| April 16, 2018As you can see from the data, driving speed makes very little difference in the overall time. Faster driving results in less driving time but more charging time (which is my preference).

Arriving with 1 mile remaining is optimal. So if you are driving and will arrive with more than 1 mile, driving faster will shorting your overall trip time - a little.

What will REALLY shorten your trip time? Larger battery. Skipping superchargers that are off your route helps too.

Rocky_H| April 16, 2018Yes, this has been analyzed in pretty good detail over the years. Here is a page showing some of the data on this, and travel time does have a sweet spot around the 75 to 80 mph level.

https://teslaowner.wordpress.com/2014/12/09/driving-charging-time/

https://teslamotorsclub.com/tmc/threads/optimum-supercharger-driving-spe...

People are sometimes oversimplifying when they just say, "As long as Supercharging speed is faster than driving speed, then drive faster!" There really is some fixed overhead time that is not in that equation for every interruption of having to come and go to a Supercharging site. The 35mph down the exit ramp doesn't change. The time sitting at the red stop lights doesn't change. Turning down a couple of side streets and pulling into the parking lot doesn't change. Backing the car up to the stall doesn't change. And then doing those steps in reverse order to get back on the highway, etc. etc. That's a little hard to model in equations, because it's a bit different for each site, but having the speed so high that you have to hit an extra stop or two can add too much extra time off the highway.

swattsw62| April 19, 2018Rocky_H, I agree with you. Too many of the responders have ignored reality, but that is common among the population lately on many subjects. I have a model S 85D and when traveling on "normal" terrain which for me always involves wind, (head or tail), hills or mountains (up or down), and more traffic than I like, the most efficient speed I have is around 70 mph. (a little loss in efficiency but less number of stops and charging time) My wife proves me right many times when she does her preferred 75+ or 80+ driving (depending on speed limits). Charging rate is not a good predictor of efficiency. I usually stop charging also when the rate drops below 300mph and that seams to help on overall trip times.

sp_tesla| April 19, 2018Efficient for car or driver?

worldtraveller| April 19, 2018Gixxxerking,

What is "lifetime efficiency"? I understand Wh/mile, but don't understand the term in %.

Bighorn| April 19, 2018@worldtraveler

I took it to mean lifetime Wh/m relative to the rate constant for rated miles which is 278 for the 85 kWh models.

Optimum speed would have to be adjusted for headwind or net climbs. I make a one for one adjustment in speed for headwind MPH when it matters, which is rarely anymore.

Darthamerica| April 19, 2018@Worldtraveler, I'm referring to my actual Wh/mi vs the rated Wh/mi. For my car I'm getting 320 vs the claimed 310. However I have the 21" wheels and tires and the fine print estimated a ~7% penalty so I'm pretty spot on when charging efficiency is considered. I always set AP to whatever the posted speed limit says, usually 65mph. Going any faster makes little to no difference in my view.

Bighorn| April 19, 2018@Gixxxer

I’m not aware of any as high as 310. The 85 requires 300 to add a rated mile but one uses the consumption figure of 278 for efficiency calculations. These rate constants are derived by looking at kWh used vs rated miles consumed.

Bighorn| April 19, 2018I guess 310 was a typo based on the math.

Darthamerica| April 19, 2018@Bighorn for P90D it's 310 rated

Bighorn| April 19, 2018That would be 97% efficient, but like I explained, not the proper way to calculate it.

Darthamerica| April 19, 2018@Bighorn you're misunderstanding. 320 is driving efficiency. It's precise and comes from over 55k miles worth of data. The -7% also includes losses due to charging efficiency. 310 is what Tesla says you get for driving efficiency with a Model S. So during driving it's ~3-4% less than advertised when measured with the 19" wheels and tires which is pretty close to what's expected.

Bighorn| April 19, 2018Where does Tesla say 310? It may be close to calculated if the battery provides 83 kWhs for 270 rated miles.

I think the Monroney sticker would show larger charging losses—I remember 38 kWh/100 miles for one model and I can’t imagine a P90 D is any better. Remember there’s about a 15% loss from the wall.

Darthamerica| April 19, 2018It used to be on the site when that car was available for order. It's also shown on the efficiency graph. I'm not talking about charging efficiency at all.

JayInJapan| April 20, 2018Bighorn’s rated numbers are much closer to reality (about 172 Wh/km). My real numbers are a bit better than @Gixxxer‘s—about 193 Wh/km), but those tend to rise with the temperatures. ;-)

focipio| August 27, 2018For practical purposes, the faster you drive the quicker you get there - assuming the same number of stops.

But this is certainly not true when taken to the extreme. Hypothetically, say it was possible to drive a thousand miles per hour and there were charging stations every couple miles. Due to drag, you would need to stop to charge every 15-20 miles, thus slowing down your trip considerably. Wind resistance grows exponentially as you drive faster. e=1/2mv2. 1,000 mph = 450m/s and 450 * 450 = 202,500. Comparatively, 67mph = 30m/s and 30 * 30 = 900. Thus driving 67mph uses 225 times less energy than driving 1,000mph. At 67mph, you can drive for ~225 minutes. At 1,000mph, you can drive for about 1 very fun minute, in which you will travel just under 17 miles. Tip: Don't drive with a consumption rate that exceeds the Supercharger rate and stop at stations that are not too far out of the way. Math is fun. Safe travels.

p.c.mcavoy| August 27, 2018focipio | August 27, 2018

“ ... Due to drag, you would need to stop to charge every 15-20 miles, thus slowing down your trip considerably. Wind resistance grows exponentially as you drive faster. e=1/2mv2. ...”

——————————————

Ehh ... conceptually yes, but your mathematical formulation is flawed ...

E=1/2mv2 has nothing to do with aerodynamic drag. It represents the kinetic energy of a body in motion and is equally valid for an object traveling through a very dense medium like undersea or in a total vacuum such as a Starman’s original roadster now headed towards Mars and the subsequent asteroid belt.

Aerodynamic drag is expressed by 1/2*rho*v2*Cd*A where

rho represents the density of the medium through which the body is traveling

Cd represents the drag coefficient

And A represents the equivalent Frontal Area of the vehicle

While the kinetic energy is meaningful relative to the energy required to bring the body to motion, it has nothing to do with the energy or work required to keep the body in motion.

So your fundamental premise that speed kills is valid, you may want to rethink your math approach the next time.

Mathew98| August 27, 2018I'm sure there's enough room for two nerds per discussion. For the sake of simplicity, 69 is the magic number.

Feel free to travel at each specific speed for 10 miles at a time and record the efficient for each one and compare.

ST70| August 27, 201846.58 MPH

schwarzkov| October 16, 2018while 75~80mph might be the most overall time efficient .. thought should be given for if you'd rather be in your car or at a bar waiting for your car to charge

seanreynoldscs| December 28, 2018I’m doing this now.

I would believe it might be around 70mph or 80mph.

Part of the equation people might be missing is, charge time increases as you drive faster because your efficiency decreases.

You could potentially minimize charge time if you drive 45 mph but then your drive time goes through the roof.

Finally in curious if stopping more frequently is beneficial because the charging seems faster when your batter is between 30 and 60 percent.

I’d imagine that if you’re going cross country it might be about 75mph and stopping at every charging station for around 20min.

Does this sound right?

Bighorn| December 28, 2018@sean

It’s about 80 in the Model S but much faster in the Model 3 because the charge rates are much higher. The new 24 hour record in Germany averaged about 68 MPH in the 3 while the S is tough to get past 55. They were doing autobahn speeds. It can’t be slow like 45 because your average is necessarily less than that. You definitely want to do short, fast splash and dash style legs to stay out of the charge taper. I’m guessing optimal speed is north of 100 MPH.

jjgunn| December 28, 2018I did this today....I started our trip at 69 MPH but once I got on I-5, I set it to 75 MPH & said eff it.

https://forums.tesla.com/forum/forums/road-tripbay-area-vegas

You definitely will use more battery & stop more often to charge but make up for it in time. As proof, I was seeing the same ICE vehicles after getting back on the road. Made me laugh.....

I charged when battery was 25%-40% & received anywhere from 106 kW - 113 kW to start. Waited until charging speed dropped I to the 80's kW then stopped charging & continued on to next SuperCharger.

In an ICE I made it from SF to Vegas in 7 hours. Yes I was driving triple digits most of the way.

Today in the Model X 100D w/20" stock tires I made it in 10-10.5 hours....& We weren't even trying to be speedy.

It's really personal preference. Do you want to save battery & stops or save time? My preference is arriving quicker.

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