Rethinking long distance driving

[bwilson4web]

A recent 2,245 mile trip inspired benchmarking and generating this graph:

• Drive segments lines at 40, 60, and 80 mph intersect at zero with the charge curve.
  • Limited to 3 hours (180 minutes) of driving
  • Maximum 30 minutes of charging in 10, 20, and 30 minutes

A horizontal line from one of the three speed lines to the charge curve gives the total minutes for one driving segment, the sum of driving time and charging time. For example, driving at 80 mph for an hour:

• -60 minutes to drive 80 mph
• +8 minutes to charge for that distance at that speed
• 60 + 8 = 68 minutes for that segment
• 80 mi * (60 / 68) = 70.6 mph equivalent speed combined driving and charging

Driving at 80 mph for an hour with required charging, the block-to-block speed will at best be 70 mph or 70 / 80 = 87.5% of the EV driving speed. This leads to a more useful summary chart.

Here is a chart of block-to-block, speed efficiency:

• 80 mph has the worst block-to-block speed efficiency, 80-87%.
• 60 mph is better, 84.5-90.5%.
• 40 mph is best, 100% within the 3 hour limit.

Full Self Driving (FSD) does excellent night driving which is typically cooler and has minimum car traffic and mostly professional driven, tractor trailers. Leaving around midnight and driving for 12 hours, arrives around checkin time at a distance motel. Fortunately there are tractor trailer companies, especially moving vans, that limit their cruise speed to 65 mph. This suggests an optimal EV strategy:

• Get on Interstate in "Chill" mode at 60 mph
• When a truck passes at 65 mph, set "Chill" mode speed to 67 mph
• Stay in lane to follow truck
  • Full Self Driving keeps a safe distance behind, not tailgating
    • FSD nag keeps your eyes looking for any road debris to avoid
  • Truck serves as camouflage so other traffic avoids tailgating
  • Larger truck rear lights improves visibility to following traffic
    • minimizes a rear end collision
  • WARNING - have quality anti-rock film on windshield
• Charge at every charger on way for fastest block-to-block speed
  • Add miles to next SuperCharger plus 30-40 mile reserve

Exit and return segments can't be helped but frequent breaks are good for the human monitoring FSD at night. A lot of miles will be covered without a lot of fatigue and minimum risk.

Bob Wilson

 

[bwilson4web]

Let me explain this chart:

• At ~80 mph, my Model 3 drives for about 75 minutes before needing a 10 minute charge
  • 80 mph * (75/60) = 100 miles
  • 100 miles / ((75+10)/60) = 70.6 mph
• At ~80 mph, my Model 3 drives for about 110 minutes before needing a 20 minute charge
  • 80 mph * (110/60) = 147 miles
  • 147 / ((110+20)/60) = 68 mph
• At ~80 mph, my Model 3 drives for about 130 minutes before needing a 30 minute charge
  • 80. mph * (130/60) = 173 miles
  • 173 / ((130+30)/60) = 65 mph

There are similar lines for ~60 mph and ~40 mph. What happens is the rate of charge decreases as the duration increases. So the fastest block-to-block speed maximizes the rate of charge by using shorter charge sessions. However, this reduces the time and miles traveled by that charge.

This chart uses my Model 3 benchmark test results with these fundamental assumptions:

• maximum 30 minute charge session, as long as I care to take at a SuperCharger
• maximum 180 mile range, ~3 hours of driving

I've tested the chart by a series to trips from my Huntsville home to Brentwood SuperChargers in Nashville TN:

• FAST - typically 75 mph over the posted 70 mph Interstate speed limit
• GOOD - typically 65 mph under the posted 70 mph Interstate speed limit
• CHEAP - minimize Interstate using the State roads direct from home to Brentwood

I am a former Prius owner who did a lot of vehicle tuning and benchmarks between 2005-2019. So my Model 3 has exceptionally long lasting, low rolling resistance tires with home made, absolute aero, wheel covers. I use passenger side windows for cabin temperature control. I also use Full Self Driving for everything but the parking lots. Ordinary, untuned Model 3s would have trouble matching my chart and benchmark results but I have to pay my bills, not yours. Regardless, the pattern should be clear.

Shorter charging sessions starting from a low state of charge will give a faster, block-to-block speed. This is because the higher initial charge rates shorten the overall trip time. I use a fixed, 40 mile reserve to the next fast DC charger(*).

A more sophisticated model would include exit Interstate and return to Interstate minutes. But these are nothing we have much control over so I choose to leave it out of the model. I am only interested in the two things I can control:

• Speed between fast DC chargers
• Duration at a fast DC charger

Bob Wilson

* - If my projected reserve is less than 40 miles as I approach the next charger, I slow down early several MPH until it remains constant. This becomes more aggressive at 30 and 20 mile range. At 10 mile reserve, I find a closer, alternate charger before going further.

If the projected reserve increases over the 40 miles, I have increased speed by several MPH until it remains constant or decreases to the the 40 mile reserve. For example, coming down a mountain or a heavy tail wind.

 

[brianc35]

Bob, this is really excellent data. And very interesting outcomes. I would love to see others with other models do a similar experiment to rule out differences between cars/tech.

What are you using to collect it?

 

[bwilson4web]

Primarily, the car metrics with at least 10 miles to even out the data. I also have Tessie but it is more for trip logs.

Bob Wilson

 

[Walt R]

To me the missing piece is the different charge curves (when in miles) for different speeds. That is, you won't get the same number of '80 mph miles' as '60 mph miles' in an equal-length charging session.

I believe Tesla makes this hard to see as they report EPA miles regardless. But some additional math could convert everything to kWh to get the total drive+charge time to cover a certain distance at a certain driving speed.

Some refinements would include a time and mileage penalty for each stop (assuming they're not highway rest areas), and also your initial period will be from a full charge rather than what you fast charge to, so the first couple stops will probably be for comfort rather than optimal speed.

(I once did a back-of-envelope calculation that showed you should drive around 1/2 of your charging speed (in actual miles at driving speed, not nominal). So if the charger is giving you more than 150 mph of actual distance, drive as fast as you're comfortable, but in other situations (slow charger or high drag, like towing) you can save more time by driving slower and not waiting as long to replace energy.)

 

[Tim Thornley]

Interesting and consistent with my experience. I have done many long distance trips in my EVs over about 5 years. I have owned 5 different EV's and have learned to look at the drag coefficient of any car before I buy it if I plan to use it on the highway. Also charge curves are available for most EVs and knowing what it looks like for my cars helps me find the sweet spot. Don't forget to add some time to deviate to the charger and plug in etc, that can tip things a little toward a little longer charge.