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One of the better justifications for 70 mph benchmark

13K views 25 replies 8 participants last post by  garsh  
#1 ·
Some handy conversions:
  • 90 kph / 181 Wh/km ~= 56 mph / 290 Wh/mi
  • 100 kph / 192 Wh/km ~= 62 mph / 307 Wh/mi
  • 110 kph / 207 Wh/km ~= 69 mph / 331 Wh/mi
  • 120 kph / 229 Wh/km ~= 75 mph / 366 Wh/mi
My typical cross country speed is "I follow trucks" which tend to be in the 65-70 mph speed range. There is some minor efficiency gain from letting the truck handle part of the aerodynamic drag but the real savings are: (1) minimum tailgating, and (2) cost-effective block-to-block time.

Bob Wilson
 
#2 · (Edited)
I see lots of articles about the most efficient highway speed for both gas and electric, but like Doc Brown says, they're not thinking fourth dimensionally: I haven't seen any that apply the difference in driving time to the numbers. So yes, driving 56 mph everywhere to get peak wh/mi efficiency may be wasting a lot of energy by going too slow!

Edit: Decided to pull out a calculator and test that. I don't know if this is accurate due to awful math skills.

Edit2: Bad math crossed out, but the point above is still important.

- 70 mph is 20% faster than 56 mph. Therefore a 5 hr trip will take 20% less time, or 1 hr less = 4 hr.

- In 5 hrs at 56 mph, the above numbers will use 1450 watts. In 4 hrs, they will use 1324 watts. Therefore you are indeed using less energy going faster! Plus less time = using less A/C and interior accessories as well.
 
#3 · (Edited)
- In 5 hrs at 56 mph, the above numbers will use 1450 watts. In 4 hrs, they will use 1324 watts.
I'm not sure what kind of math you were doing there. Watt is a measure of power - Watt-hour is a measure of energy.

Going 5 hours at 56 mph means you've traveled 280 miles. Using 290Wh/mi, for a total of 81.2 kWh
Going 280 miles at 75mph , you're using 366 Wh/mi, for a total of 102.5 kWh
 
#5 ·
Being comfortable with units like Wh/mile is not an easy thing for most. The fact that we use units like Wh instead of Joules hints at the difficulty many have in converting between energy, power, and efficiency metrics.

That said, this is a good learning opportunity.
 
#9 · (Edited)
The right term to use is energy, not power. Power is energy per unit time and is measured in Watts which is Joules / Sec. To convert to energy (used) multiply by the time span. We tend to bastardize things and multiple Watts (which is a per second metric) by hours. So ... we get the unit Watt - hours. (Wh)

The efficiency we all talk about with EVs is measured in the amount of energy used per mile. Energy/mile .... Wh/mile. This elegantly deals with all the power/ speed / time / distance math.

So, if the most efficient speed is 30 mph ... which translates to X Wh/mile ... you can determine the amount of energy for the trip by multiplying the miles times the efficiency measure. In this case, for 100 miles, the energy used is 100X Watt Hours.

There is no time component to the ‘break even’ as it has already been taken into account. It all comes down to the Wh/mile at a different speeds .... as Garsh is showing. Very cool how simple it is.
 
#11 ·
@garsh That chart above is a little meaningless without knowing whether the curve applies to time or distance, and depending upon which, how long and how far. I'm guessing it applies to 1 hour of driving?

The practical reason for understanding efficiency and these curves is either, what's the maximum distance I could go, or what is the least amount of time I could take to get somewhere. For example, if you want to travel 620 miles/1000km in the least amount of time, is it more time efficient to drive at 62mph/100kmh and charge once, or 75mph/120kmh and charge twice. Obviously that partly depends on how quickly you can charge etc.
 
#13 ·
@garsh That chart above is a little meaningless without knowing whether the curve applies to time or distance, and depending upon which, how long and how far. I'm guessing it applies to 1 hour of driving?
I'm not sure what you're asking.

The last graph I posted is a graph of power (measured in Watts). Power is a rate of energy used per unit time. It applies no matter how long you are driving.

If you want to find out how much energy you used (measured in Watt•hours), you take that power and multiply by the amount of time at which you were drawing that much power.
 
#20 ·
My understanding of Bjørn Nyland's "Sweet Spot Speed" video is to find the shortest time to cover a certain distance. Taking into account an EV's efficiency plus charge time. It's an interesting reference which could help first time EV buyers. How long will it take an EV model to cover a certain number of miles including recharging to 80%.