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Same 324 mile drive - huge difference in efficiency

Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
Since I got my car in March 2020 I've traveled a few times between my home in Frisco, TX and Fayetteville, AR and know pretty well when and where to stop to make it a pleasant drive. Going through Oklahoma takes some careful planning but there are few good Chademo chargers along the way and A Better Route Planner is helpful.

On my recent trip back from Fayetteville yesterday I got in major trouble and I had to make an emergency stop to charge and still only made it back home with 2 percent left. The super low efficiency caught me off guard. The energy graph has impressed me many times on how accurate it is at predicting usage but this time it was off the charts. Every few minutes it kept dropping another percent.

Here are some stats from TeslaFi:
Distance 324 miles
Altitude loss 739 ft
110.89 KWH used for an Wh/mile of 361
Average speed 58 mph
Outside temperature between 86-98 F and no precipitation
light wind between 10-13
SW 2021.12.25.7

In comparison the opposite direction driven the day before with at least 200 lbs of additional weight (moving my girl to college) used 93.49 KWH (296 Wh/mile) with an outside temp of 88 F, That is a difference in efficiency of 22%.

Another point of reference is my last trip back from Fayetteville in May:
Same distance and altitude loss
88.99 KWH used for an Wh/mile of 269
Average speed 62 mph
Outside temperature between 71-76 F and no precipitation
light wind between 10-15
SW 2021.4.15

The difference in efficiency is 34% for the exact same trip and that does not make sense to me.

My lifetime average is 293 Wh/mi over 14,470 miles.

Any idea what is going on?!?!
 

Rice&Curry

Member
May 9, 2018
381
196
San Jose
I can only think of tire pressure as one of the factors, but that cannot account for 34% difference in energy usage- not sure what other factors we are missing ?
 

jcanoe

Active Member
Oct 2, 2020
3,429
3,481
Maryland
Check that you don't have a stuck brake caliper. This would create additional drag and also heat up the brake rotor. Drive for a bit and then hold your hand close to (but not touching) the brake rotors. If one brake rotor feels warmer that the other three calipers then you may have a stuck caliper.
 

Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
Check that you don't have a stuck brake caliper. This would create additional drag and also heat up the brake rotor. Drive for a bit and then hold your hand close to (but not touching) the brake rotors. If one brake rotor feels warmer that the other three calipers then you may have a stuck caliper.
Good idea. I'll check it out. Looking at them still makes me happy. So glad I painted them :)
 

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Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
Tail wind versus headwind?

Less AC on the 2nd trip?
Yeah, I'm sure those can be factors. It was a lot warmer yesterday than back in May and I can't tell from TeslaFi in what direction the wind blew. Just that it was a similar range, 10-15. But 34% difference in efficiency? Perhaps I don't know my car that well still.
 

Exelion

Member
Feb 21, 2021
257
336
Los Angeles, CA
It's the AC.

What's the interior of your car set to? If it's 71, then on one trip the AC is hardly running, and on the other, if it's as high as 98 outside, the car's HVAC will be working a LOT to keep it down at 71 for you. To give you an example, when I go grocery shopping, I keep the AC on in the parking lot set at 71. When it's 98 outside, for 30 minutes, I'll come back to a car 5-6% lower SOC than when I got there.

Next time, (if it's bearable) try running without the AC, maybe with just the fan on (no compressor) and crack a few windows open. See what you get.
 
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Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
Agree and I’m also convinced the outside temperature is a factor. I’m still in shock that the difference can be so large as 34% if that is the case.

Why isn’t the efficiency graph estimating that difference correctly at the start of the trip? Tesla being a company talking up AI/ML all the time should be able to gather the necessary data and make a better prediction based on outside and inside temperature. The tool already estimate consumption based on speed and elevation changes. I almost got stranded because Tesla had no clue what my car was consuming because of the HVAC. And easy fix in my mind.
 
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Exelion

Member
Feb 21, 2021
257
336
Los Angeles, CA
Some other EV's have an info page with actual energy consumption of the devices as they happen. It would be nice if Teslas did the same - I don't think it's difficult to do, especially how much the HVAC (which runs off the main traction battery, not the 12V!) consumes, especially at max.

TBH, I'm not sure there's a post or a thread about how much the HVAC consumes per hour of use at full cold blast. I'd like to know.
 

garrett5688

Member
Oct 7, 2017
611
1,096
DFW
Also vote for AC. It’s an energy hog when you’re trying to keep a 30+ degree difference from ambient. Also, make sure to use recirculate, it will help. Sometimes just setting to “auto” sucks in outside air longer than it should and uses way more energy.

which chademo chargers do you stop at between Frisco and Fayetteville?
 
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Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
I'll try recirculating and see if it makes a difference.

ABRP was set to 95 F and it has been good at predicting charge times and consumption previously but it was also caught off guard.

Here is my temperature efficiency graph from TeslaFi. It show a 10% reduction in efficiency at 95-100 F that was the temperature yesterday compared to my last trip in May that was 70-75 F. So temperature matters for sure but is wind the other 24%? I have still to check if a caliper is stuck.
chart_main.jpeg

Anyone else have a similar TeslaFi graph to share? Perhaps my data is off.

I usually stop at Rover's Taxi in McAlester. Also done the MacDonald's off Indian Nation Turnpike but I prefer the food options downtown. ABRP usually recommend a 45 min stop on the way back but as I arrived way low I ended up staying 1h 12 min to get 45.4 kW. Chademo is painfully slow...

My emergency stop ended up in being in Durant. Can't really recommend it. EVGateway said the Chademo port was unavailable but when I used a credit card it showed up as available. The charger cut me off twice after about 5 minutes so I had to restart it.
 

golferguy

Member
Jan 30, 2021
87
91
SE Florida
Fayettville AR to Frisco TX yesterday had a 14 MPH wind from South. For the past week, winds were from the South 7-17 MPH. So going from Tx to AR you had a good tailwind and in the return trip, a good headwind.

I live in Florida where it's flat for the most part and AC most of the year. When I have a tailwind in my M3, at 80mph I get as good as 265 wh/m but can be as bad as 315 wh/m with headwinds. 50 wh/m difference. Although yours a bit more pronounced but the Y is a larger vehicle.
 

Exelion

Member
Feb 21, 2021
257
336
Los Angeles, CA
I usually stop at Rover's Taxi in McAlester. Also done the MacDonald's off Indian Nation Turnpike but I prefer the food options downtown. ABRP usually recommend a 45 min stop on the way back but as I arrived way low I ended up staying 1h 12 min to get 45.4 kW. Chademo is painfully slow...
How long do you stop for at each stop, and when you do, are you running the AC while outside the car? Or is the car's HVAC turned off and the inside temperature rising to 105 degrees?
 

Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
How long do you stop for at each stop, and when you do, are you running the AC while outside the car? Or is the car's HVAC turned off and the inside temperature rising to 105 degrees?
Yeah it went a lot higher than that. During my 1+ hour stop in McAlester I think I hit north of 130F. I remotely turned on the HVAC while walking back to my car so it dropped below 90 or so before I left. During my shorter stop in Durant it was on the whole time. But for sure, HVAC was working hard but that is not unusual this time of the year. I might be wrong but I think it should be reflected in the temperature efficiency from TeslaFi. It is 17 month of statistics so it is not the first hot day.
 

Exelion

Member
Feb 21, 2021
257
336
Los Angeles, CA
Over 130? That’s where I was leading to: the part I’m not able to answer and hopefully someone else could (because I also want to know)

-is it better to leave the AC on cooling the car or stop it, let the car heat up to 130+ and then run the AC before continuing the drive? Or is it the same efficiency?

I realize hot temps are sometimes unavoidable, if its a hot day, its a hot day no matter what you try to do, and efficiency will suffer regardless.
 

Zoomit

Active Member
Sep 1, 2015
2,255
4,370
SoCal
The difference in efficiency is 34% for the exact same trip and that does not make sense to me.
So temperature matters for sure but is wind the other 24%?
The difference in energy consumption between 60 mph with a 10 mph headwind and 10 mph tailwind is ...runs calculations... ~36%.

Of course, you didn't have a straight-on headwind and tailwind, and the different ambient temperatures were definitely a factor, but opposite winds can easily cause greater than a 20% difference in consumption.
 

Y_4_speed

Member
Mar 2, 2020
45
37
Dallas
The difference in energy consumption between 60 mph with a 10 mph headwind and 10 mph tailwind is ...runs calculations... ~36%.

Of course, you didn't have a straight-on headwind and tailwind, and the different ambient temperatures were definitely a factor, but opposite winds can easily cause greater than a 20% difference in consumption.
I see your point. How did you get to 36%?

Here is my lifetime stats of Wh/Mile and I can't vouch for how much of this has been with headwind or tailwind but the difference is about 10% on average for a +/- 10 mph difference.
chart_main (2).jpeg


So adding up what I've gathered so far I may have lost 10% because of temperature and another 10% because of wind. That leaves me 14% not accounted for. Maybe I'm overthinking this but it sucks. It is a big number, especially when good charging stations are far in between and I want consistency.
 

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