Inefficient trip

[Austindude]

A 20 mph headwind is effectively the same as adding 20 mph to the speedometer. And drag increases exponentially, that's a heck of a drain on the battery.

You need to learn to be comfortable letting the battery go to 5%, take a drive and get then numbers low. Even 5% is a lot of miles. It's so much fun to watch it get down to only a few miles remaining. And it's a good thing for people to get used to.

Hey I'm laughing here, you're right I must be a wimp worrying about arriving with 15% charge. Anyway the update is I made the treturn rip back to Austin today with no noticeable wind. I started at 93% charge in McAllen and drove a solid 80 mph and made it to Three Rivers with 35% charge - which for a wimp like me felt pretty damn good. No wind and the temp outside was probably mid 70's for that part of the drive home with some fog and drizzle along the way. The point I was trying to make with the original post was that here on a trip I make quite often I was surprised when conditions really impacted my drive efficiency. Shouldn't have been surprised.

 

[M3 Cowboy]

My wife and I often make a trip to McAllen TX from Austin TX in our LR RWD Model 3. I have 5.15 software, 18 inch tires, third party wheels and tire pressure is set to around 42 to 43 psi. Charge to between 95% to 100% at home, go about half way to Three Rivers TX supercharger, charge back to around 90 to 95% and easily make it to McAllen supercharger at the convention center. No sweat or range anxiety ever until this trip. Second half (which is a little more than half way) of the trip we usually drive 80 mph with the AC set to 74 degree. No big deal. But this time we had a strong headwind all the way from Three Rivers to McAllen and the efficiency dropped from what is normally 250 Wh/mile or less to about 365 Wh/mile. Wow, we went from 93% charged to 15% charged when we got to McAllen and in fact that only happened by reducing the speed to 75 mph (which was the speed limit) instead of 80 mph. Climate set to 76 degrees to reduce our energy load. This is the first time that I have had the car (about a year) that I actually was starting to get concerned that we might really need to slow down to make it. Overreacting for sure, but the navigation kept dropping our predicted battery arrival percentage all along the trip. I've never arrived with anything less than 25% previously. The point is that after taking this trip many times and feeling so confident I had to rethink things. Sure glad I wasn't in my old S 70 or in a MR or standard Model 3.

Hi Austindude,

I also have the LR RWD M3.

I often drive between San Antonio, and Corpus Christi, TX and I have also encountered some major headwinds. The headwinds are the worst when heading towards Corpus between the Three Rivers Supercharger and Corpus Christi. I once charged up to 260 miles of range and arrived home with only 25 miles left. I was shocked since the drive is only 145 miles. What I do know is that on that trip the winds got up to at least 40 mph gusts and 25 to 30 sustained. I was hitting about 385 to 405 Wh/mile. Of course I didn't realize the worst of range reduction until about 20 minutes past three rivers. I shut down all non-essentials and made it luckily. As you know, this route is not one you want to be stuck on. It's very dark and unforgiving terrain with nowhere to charge outside of the superchargers on this particular stretch. I for sure felt the range anxiety on that one!

 

[stayfocus18]

My drive to stl mostly 65-70 I've seen my usage around 310-330.... drove her down to 1% lol... I was a little scared but it was 20-40 degrees F most of the drive... Still handled it like a champ. I occasionally turned on the HVAC to clear the windshield and kept the seat heater on. Like OP faced a lot of headwinds. When I was driving through PA, OH There was a snowstorm but the car kept pushing without a problem.

 

[Lunares]

Ughh.

b^x is an exponential function
Aero drag is a quadratic function of the form v^2

If we assume a 20 mph headwind, OP's description of driving 75 mph with a headwind instead of 80 mph without implies (95/80)^2 more aero drag, about 41% more drag.

Drag increases with v^2 but power to overcome drag is more complicated.

Drag (physics) - Wikipedia

In fact power used to overcome drag is a function of velocity cubed. So going from 65mph to 75mph uses (75/65)^3 =50% more power.

But overcoming a wind does scale with squared, but your formula isnt quite right. Due to a headwind you lose (v_hw+v_car)^2/v_car^2

So combining the two, (v_hw+v_new)^2/v_new^2 * v_new^3/v_old^3 = (v_hw+v_new)^2*v_new/v_old^3

= 32% more power (and therefore energy) for dropping 80 to 75mph in a 20mph headwind

 

[Matt L]

My wife and I often make a trip to McAllen TX from Austin TX in our LR RWD Model 3. I have 5.15 software, 18 inch tires, third party wheels and tire pressure is set to around 42 to 43 psi. Charge to between 95% to 100% at home, go about half way to Three Rivers TX supercharger, charge back to around 90 to 95% and easily make it to McAllen supercharger at the convention center. No sweat or range anxiety ever until this trip. Second half (which is a little more than half way) of the trip we usually drive 80 mph with the AC set to 74 degree. No big deal. But this time we had a strong headwind all the way from Three Rivers to McAllen and the efficiency dropped from what is normally 250 Wh/mile or less to about 365 Wh/mile. Wow, we went from 93% charged to 15% charged when we got to McAllen and in fact that only happened by reducing the speed to 75 mph (which was the speed limit) instead of 80 mph. Climate set to 76 degrees to reduce our energy load. This is the first time that I have had the car (about a year) that I actually was starting to get concerned that we might really need to slow down to make it. Overreacting for sure, but the navigation kept dropping our predicted battery arrival percentage all along the trip. I've never arrived with anything less than 25% previously. The point is that after taking this trip many times and feeling so confident I had to rethink things. Sure glad I wasn't in my old S 70 or in a MR or standard Model 3.

250 Wh/mile at 80mph...all downhill?!?!

I can see getting that at 60mph...not 80

 

[lolder]

M3 LR RWD 250 wh/m at 70 mph. 15% SOC is the same as "Empty" on most ICE cars. You should understand if the car shuts-down due to low SOC it's a flatbed tow that Tesla won't pay for. There have been cases where older Teslas have shut down way before 0% SOC. Planning on arriving with 5% is bad advice and traffic tie-ups or bad weather can put you in jeopardy of a tow.

 

[SageBrush]

= 32% more power (and therefore energy) for dropping 80 to 75mph in a 20mph headwind

Your comments are why I calculated the increase in drag.

Calculating the increase in energy/distance works out to a different percentage because the cars are traveling at different speeds

Starting from
Work = Force * Speed
It is apparent that the energy/distance increase is not the same ratio as the increase in Aero for two reasons:
1, the speeds are different,
2, the total force increase includes components that do not scale with speed.