Efficiency at 70 vs. 75 vs. 80 on highway?

[xycarp]

My truck is coming soon…. Totally bought in, but not crazy about the idea of driving 70 on the highway. All the range tests I’ve seen seem to focus on that specific number.

Any good examples of range and impact of driving at 80 mph? Specifically for an ER Lariat.

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[Maxx]

If you are not worried about running out of juice on a long leg of a trip, just drive 80 and don’t look at the numbers. Even if you get 1.8 mi/Kwh instead of 2, it is still a lot cheaper than gas charging at home.

 

[Roy2001]

70mph means ~2 miles per kWh, 75mph is ~1.8 miles per kWh, 80 mph is ~1.6 miles per kWh.

 

[sotek2345]

70mph means ~2 miles per kWh, 75mph is ~1.8 miles per kWh, 80 mph is ~1.6 miles per kWh.

Converted to range, that would be ~260 miles at 70mph (lines up with most testing, maybe a little low), 235 miles at 75mph (the little testing I have seen says this is about right, but maybe a little high), and 210 miles at 80mph. Cut ~30% off in the cold.

In general, given the Lightning's relatively flat and fast charging curve, drive at whatever speed you are comfortable at and gets you to the next charging station with a comfortable buffer.

In our Mach-e, I generally start out with the cruise set to 72, but then start bumping it up part way through the trip when I am sure I have a safe range buffer.

 

[RickLightning]

70mph means ~2 miles per kWh, 75mph is ~1.8 miles per kWh, 80 mph is ~1.6 miles per kWh.

Assuming those are accurate, that's a 10% drop in miles per kWh from 70 to 75, and another 11% drop to 80, and in total a 20% drop going from 70 to 80mph.

Let's put this into MONEY terms.

If you drive 500 miles at 2 miles per kWh, you'll use 250 kWh of electricity in the truck. Assuming ~ 10% loss from DC charger to vehicle, that means you need to purchase 278 kWh. Assuming EA Pass+ rates of 31 cents plus tax in most states, that's $86.18, plus tax, to drive 70.

At 80mph, you'll use 500/1.6 = 312.5/.9 = 347 kWh x 31 cents = 107.57 plus tax.

So you'll pay $21.39, plus tax, to drive 10 miles faster for 500 miles. You'll get there 53 minutes sooner - 6:15 vs. 7:08 total time spent.

Is 53 minutes worth $21.39 plus tax, call it $23?

Edit - it was rightfully pointed out here that I did NOT include stop time in this calculation. BIG OMISSION.

 

[RavenYZF-R6]

Is 53 minutes worth $21.39 plus tax, call it $23?

Yup!

 

[Roy2001]

Assuming those are accurate, that's a 10% drop in miles per kWh from 70 to 75, and another 11% drop to 80, and in total a 20% drop going from 70 to 80mph.

Let's put this into MONEY terms.

If you drive 500 miles at 2 miles per kWh, you'll use 250 kWh of electricity in the truck. Assuming ~ 10% loss from DC charger to vehicle, that means you need to purchase 278 kWh. Assuming EA Pass+ rates of 31 cents plus tax in most states, that's $86.18, plus tax, to drive 70.

At 80mph, you'll use 500/1.6 = 312.5/.9 = 347 kWh x 31 cents = 107.57 plus tax.

So you'll pay $21.39, plus tax, to drive 10 miles faster for 500 miles. You'll get there 53 minutes sooner - 6:15 vs. 7:08 total time spent.

Is 53 minutes worth $21.39 plus tax, call it $23?

Your math does not count charging time.

Quite a few EV owners mentioned that driving above 80mph means more time for the entire trip due to extra charging trips/time.

 

[LightningShow]

My truck is coming soon…. Totally bought in, but not crazy about the idea of driving 70 on the highway. All the range tests I’ve seen seem to focus on that specific number.

Any good examples of range and impact of driving at 80 mph? Specifically for an ER Lariat.

On a road trip, it's faster to drive fast and charge more. The reality is you almost never go 80mph straight out for hours. I drive 80 all the time on my commute to work but occasional slowdowns and the 25% portion of the trip on surface roads means my efficiency for the trip is 2.5mi/kwh despite driving 75-85mph whenever the conditions allow it. IMO it has to be damn near impossible to get under 2 mi/kwh when you average out all of your driving unless you live and work 100 yds from the expressway and there is absolutely no traffic where you live. (ETA: Assuming good weather conditions, lots of HVAC use will knock you down lower)

I went on a roadtrip last week and I left at 5am in both directions so I could knock out as much as possible without traffic. I was driving 78-80 almost the entire first leg of each trip (150-200mi) and I still ended with 1.9mpk going south and 2.0mpk going north.

 

[LightningShow]

Your math does not count charging time.

Quite a few EV owners mentioned that driving above 80mph means more time for the entire trip due to extra charging trips/time.

That's incorrect. Maybe it just feels longer to them because they are stopped longer. The DCFC add back charge at a much, much faster rate than you expend it. That's why it's faster to drive fast and charge more.

 

[sotek2345]

Assuming those are accurate, that's a 10% drop in miles per kWh from 70 to 75, and another 11% drop to 80, and in total a 20% drop going from 70 to 80mph.

Let's put this into MONEY terms.

If you drive 500 miles at 2 miles per kWh, you'll use 250 kWh of electricity in the truck. Assuming ~ 10% loss from DC charger to vehicle, that means you need to purchase 278 kWh. Assuming EA Pass+ rates of 31 cents plus tax in most states, that's $86.18, plus tax, to drive 70.

At 80mph, you'll use 500/1.6 = 312.5/.9 = 347 kWh x 31 cents = 107.57 plus tax.

So you'll pay $21.39, plus tax, to drive 10 miles faster for 500 miles. You'll get there 53 minutes sooner - 6:15 vs. 7:08 total time spent.

Is 53 minutes worth $21.39 plus tax, call it $23?

You won't get there 53 minutes earlier either. At 70mph, you can make 500 miles with 1 charging stop (if you push it back up near 100) or 2 short stops right in the meat of the charging band. At 80mph, you will need 2 long charges, maybe 3 if you hit any weather or elevation changes. Best case you save ~20 minutes.

 

[swesson]

My truck is coming soon…. Totally bought in, but not crazy about the idea of driving 70 on the highway. All the range tests I’ve seen seem to focus on that specific number.

Any good examples of range and impact of driving at 80 mph? Specifically for an ER Lariat.

As an old Tesla owner (2015), there was a common saying..."slow down to get there quicker". Slower speeds means more range = fewer/shorter charging stops. I found 80mph is just not a good tradeoff of speed and time and charging.

 

[Tony Burgh]

Aerodynamic drag (resistance to flow or particle movement) is proportional to velocity squared.
@70 say R=1
@75 R=(75/70)^2 times R @70 = 1.15 R or 15%more drag
@80 R=(80/70)^2 times R@70 = 1.31R or 31%more drag

Another way to look at it is energy required to go distance D
@70 say D=1
@75 D=D70/1.15 or 87% of D@70 but you cut 4 minutes per hour
@80 D=D70/1.31 or 77% of D@70 but you cut 7 1/2 minutes per hour

But at higher speeds you may not make the distance D.

Not rocket science, aerodynamics

 

[LightningShow]

As an old Tesla owner (2015), there was a common saying..."slow down to get there quicker". Slower speeds means more range = fewer/shorter charging stops. I found 80mph is just not a good tradeoff of speed and time and charging.

Have you ever run the numbers on that? Because they say the opposite. With 50kW charging infrastructure it may well have been true but with 100kW+ chargers the charge time is fast enough to offset the lost time to get the charger and back to the road.

 

[MM in SouthTX]

The calculations that people give you for mileage need to be qualified based on traffic. Mileage varies DRAMATICALLY with traffic. On open roads with no traffic you will get FAR lower mileage than on a turnpike with all the traffic moving as a group. Especially at 80. And if there's a headwind? It's worse.

 

[p52Ranch]

Aerodynamic drag (resistance to flow or particle movement) is proportional to velocity squared.
@70 say R=1
@75 R=(75/70)^2 times R @70 = 1.15 R or 15%more drag
@80 R=(80/70)^2 times R@70 = 1.31R or 31%more drag

Another way to look at it is energy required to go distance D
@70 say D=1
@75 D=D70/1.15 or 87% of D@70 but you cut 4 minutes per hour
@80 D=D70/1.31 or 77% of D@70 but you cut 7 1/2 minutes per hour

But at higher speeds you may not make the distance D.

Not rocket science, aerodynamics

Drag is proportional to velocity squared but power required to overcome that drag is proportional to velocity cubed. Going form 70 to 80 the power required to overcome the aerodynamic portion of total vehicle drag is increased by a factor of 1.49.

We can only calculate the increased power consumption if we know all of the components of drag on the vehicle. The empirical evidence seems suggest that between 70 and 80 mph aerodynamic drag is approximately 50% of the total vehicle drag.

I've also seen approximately 2.0 miles per KwH at 70 and 1.8 at 75. I haven't tried 80 yet. This on a SR Lightning.

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