Efficiency / Speed / Drag

[Scottie’s MachStang]

I've stumbled across some awesome stuff in the archives and it has me wondering if anyone has seen a graph that shows speed vs aerodynamic drag.

For example, I see a marked improvement in efficiency (as much as 30%) when I drive slower on the "highway" portion (across the Sunshine Skyway) which is about 1/3 of my commute. The speed limit is 65, passing lane traffic comfortably runs around 78.

I am wondering - other factors excluded - what the most efficient speed might be.

I anxiously await the advice of the collective brain trust.

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[RickMachE]

The most efficient speed is zero...

Yes, with any vehicle (no matter the fuel type), if you drive 65 on the highway, or slower, you'll get better efficiency. The tradeoff is taking longer to get there vs going faster and having to charge more often.

People have analyzed driving faster and charging more vs. driving slower and charging less, and as I recall came up with 65 as the "ideal".

Many find that 65 works for them. It doesn't for me on most trips. I do around 73 in the Mach-E, and around 70 in the Lightning.

Also, keep in mind that a headwind (or weather) will dramatically impact your efficiency. Going 65 into a 20mph headwind is the same as driving 85. I got to experience this last fall in Arizona, a very strong wind came up, all the truckers slowed down. I watched This Trip drop from 2.7 to 2.1 very quickly. Made the hotel with 4 miles to spare.

 

[bbulkow]

I've stumbled across some awesome stuff in the archives and it has me wondering if anyone has seen a graph that shows speed vs aerodynamic drag.

For example, I see a marked improvement in efficiency (as much as 30%) when I drive slower on the "highway" portion (across the Sunshine Skyway) which is about 1/3 of my commute. The speed limit is 65, passing lane traffic comfortably runs around 78.

I am wondering - other factors excluded - what the most efficient speed might be.

I anxiously await the advice of the collective brain trust.

If by energy efficient, there is a speed which is greater than zero and less than 65 which is the lowest kwh per mile. You can see at 40 mph, in the efficiency graph, numbers like 7.5 and 9.5 and higher for city driving. I imagine that friction losses take over at the very low end.

ABRP occasional has suggested I drive really slow to make a long stretch without a charger. I think I put in a drive across Route 50 to Moab and it suggested a stretch like that. As new chargers go in, not sure if it still would. I believe ABRP allows you to put in a "nominal speed" for planning, and seems to have some speed / efficiency curve, perhaps the simple theoretical drag - speed - squared one, perhaps one driven by practical data?

In terms of time efficiency, is it faster to speed then charge on a road trip, it would seem to highly depend on the charger, no? For example, if were going to a reliable L2 charger (eg, at your house or if you have a hotel reservation), going at as high a speed as gets you there without stopping elsewhere would certainly seem the best. If you're in the middle of the day, it might also depend on your target charger - a 50kwh charger vs a 350kwh charger might change the math a bit. So would a 15 minute wait for an available charger, vs going a bit slower and being able to go from 3 charges in a day to 2? Or, similarly, if you know you're going to stop at charger X, is it better to get there 3 minutes faster but spend 2 minutes charging getting those extra electrons back? Faster might be a win - up to a point.

I've rather settled on liking my cruise control set to 67. I don't have good data for exactly that speed, but it seems to balance range and desire to get there, just looking at the onboard efficiency numbers.

Higher range tricks - when one needs them - seem to be climate control down, speed down, drafting up. Battery temperature matters too - but it's harder to influence. Precondition and departure times. Also arranging your drive to the warm parts of the day? Cross country on the southern route?

I seem to get an incredible efficiency finding a large vehicle and using blue cruise behind them, for the drafting. Even vehicles like trade vans and work trucks with racks or RVs. But still - drafting at 75mph and drafting at 65mph are still very different. Especially as the larger trucks, now that they have speed logs, tend to stick to 55mph. Grabbing a draft off one of those puppies and I'm sipping electrons, if I was ever in a jam getting to a charger. I do tend to draft when it's practical now, as habit.

I wish I had an "efficiency" page in CarScanner Pro where I could see a more fine grained efficiency graph!

 

[Mach-Lee]

I've stumbled across some awesome stuff in the archives and it has me wondering if anyone has seen a graph that shows speed vs aerodynamic drag.

For example, I see a marked improvement in efficiency (as much as 30%) when I drive slower on the "highway" portion (across the Sunshine Skyway) which is about 1/3 of my commute. The speed limit is 65, passing lane traffic comfortably runs around 78.

I am wondering - other factors excluded - what the most efficient speed might be.

I anxiously await the advice of the collective brain trust.

Drag is proportional to velocity squared. So going 2x as fast means 4x more drag. Slowing down makes a huge difference.

The most efficient speed depends on the idle loads on the vehicle. Especially the amount of HVAC. If you are running a lot of heat, the most efficient speed will be higher, maybe 35 MPH. If you are not running much HVAC, the most efficient speed will be slower, perhaps 25 MPH. So it depends.

 

[phidauex]

Drag is proportional to velocity squared

This is the real key here that many people don't realize. Power for acceleration, HVAC, lights and the like vary pretty linearly and are easy to understand. Drag's relationship to speed means that while 75mph doesn't sound much faster than 65mph, it is 33% more drag force, which dominates the losses at highway speeds.

 

[RickMachE]

Drag is proportional to velocity squared. So going 2x as fast means 4x more drag. Slowing down makes a huge difference.

The most efficient speed depends on the idle loads on the vehicle. Especially the amount of HVAC. If you are running a lot of heat, the most efficient speed will be higher, maybe 35 MPH. If you are not running much HVAC, the most efficient speed will be slower, perhaps 25 MPH. So it depends.

This is the real key here that many people don't realize. Power for acceleration, HVAC, lights and the like vary pretty linearly and are easy to understand. Drag's relationship to speed means that while 75mph doesn't sound much faster than 65mph, it is 33% more drag force, which dominates the losses at highway speeds.

You guys are so much smarter than me. I didn't have any knowledge of drag's impact on efficiency until I joined this forum. I have found it difficult to fit more than 2 comfortably, and my wife continues to roll her eyes at me.

Wait, wut? They're not talking about drag queens?

 

[silverelan]

This is the real key here that many people don't realize. Power for acceleration, HVAC, lights and the like vary pretty linearly and are easy to understand. Drag's relationship to speed means that while 75mph doesn't sound much faster than 65mph, it is 33% more drag force, which dominates the losses at highway speeds.

I set the cruise control to about 72mph. I can really see the efficiency difference at 65mph but my sanity can’t handle going 70 or slower.

 

[Mrn]

I live in the desert SW and have been reluctant to turn on the AC as the midday temps hit the low to mid 90s. I started experimenting with putting down the front and back passenger side door windows enough to get a nice breezy vortex going inside the car.

Then I noticed my predicted range after charging was about 10% lower than before when I was using a slight opening of the drivers window and the right rear passenger window. So evidently the drag coefficient of the Mach-e can vary significantly depending upon what you do with the windows ?.

 

[TheSteelRider]

I hear by the end of THIS YEAR our cars will be driving themselves here on earth whilst we are arranging our furniture in our Mars settlement habitat, so I'm not really concerned about drag (or even reality).

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