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Freeway Drafting in a Model 3

Freeway Drafting in a Model 3

I tested drafting 18 wheelers during my road trip (see post “ 8,000 mile roadtrip results”). I’ll walk through my process, key learnings and results.
My configuration: M3, LR, Aero wheels

My drafting process:
1) Drive in the slow lane to wait for the correct type of truck(big square 18 wheeler, good mud flaps on back wheels, driving 5 mph above the speed limit)
2) After truck passes you, pull in behind the truck, increase your speed 3 to 5 MPH above the speed of the truck(this ensures you will maintain the correct distance with the truck)
3) Set the “car follow length” between 1 to 3 car lengths
4) When the truck starts to pass a slower vehicle, slow your speed to match the drafting truck
5) Pass the slower vehicle but leave enough of a gap so you can slide in between the drafting truck and the slower vehicle

Key Learnings:
1) Don’t bother trying on busy highways because too much interaction is required
2) Attempted drafting in moderate rain but it didn’t save any Wh/mi.
3) Sometimes the truck you are drafting will have a hard time getting into the fast lane in order to pass a slow vehicle. Use your nimble Model 3 to quickly pull out into the fast lane, slow down and flash your lights to let the drafting truck to get into the fast lane. I was able to stay with one truck for over 30 minutes by using this tip. This forms a nice symbiotic relationship.

Advantage of drafting:
1) Reduced my Wh/mi by “11” traveling at 73mph over a 30-mile distance. Reduced from 246 Wh/mi without drafting to 235 Wh/mi with drafting. (Similar road, temperature and speed taken into account)

Disadvantage of drafting:
1) Increased risk of rock damage from drafting truck (ironically, I got a small windshield crack not when I was drafting).
2) Safety issue driving close to truck at high speeds. I have noticed that the Model 3 increases its following distance the faster you are going.

kevin_rf | 23. Januar 2019

3) Often the trucker driver will get annoyed and drop his speed and start driving on the rumble strip to get you to stop.

That said, a follow distance of 3 seems to be a safe value that doesn't crowd them to much.

SalisburySam | 23. Januar 2019

@rwgallah, thanks for sharing your experiences. You do know that the following distance isn’t distance at all but half second increments of time. That’s why the distance increases with speed. So “4” equates to about 2 seconds and whatever that represents in distance depends on your speed.

My takeaway from your excellent post: drafting is efficient, drafting is not the safest way to drive.

Tuning In | 23. Januar 2019

I get the fun of scientific discovery on your own. However the real world application is nil since the risks finacially doing far out weight the small financial benefits.

Magic 8 Ball | 23. Januar 2019

Is your organ donor card filled out?

foodking | 23. Januar 2019

i haven't kept track of numbers, but i've found that the closer you are, the more electricity you save. I've previously done this with my ICE cars. My 2006 jetta tdi gets 60mpg behind large trucks.

wiboater4 | 23. Januar 2019

Magic 8 Ball +1 LOL

SteveWin1 | 23. Januar 2019

I keep a tow strap connected to the front of my Tesla on long trips. I've got a grapling hook on the other end and when I see a big semi, I get behind him, turn on EAP, stick my head out the window and throw the hook at his bumper. I'm getting pretty good at it. Once you're hooked on, you can crank your speed way down and let him tow you. Your car will regen back up to full battery fairly quickly. Way better than just drafting!

ewd7 | 23. Januar 2019

Just about to roll over into mile 15000 if my 2 month "road trip", I've only drafted a couple times and it was out of desperation given the trip computer calculated the charge remaining and didn't take -14F weather into account. It made a big difference as the op said and got me to the supercharger, but I wouldn't recommend doing it on a regular basis due to the rocks, unpredictable truck drivers and danger, even with automatic braking and adaptive cruise control.

jpalermo | 23. Januar 2019

You're probably a lot safer behind a big truck with a follow distance of 1-3 than you are behind other traffic with an even larger follow distance.

Unless that truck somehow gets into a head on collision with another truck, there's not much on the freeway that can stop it quickly in an accident.

But yeah, you're probably going to take more rock damage.

Lorenzryanc | 23. Januar 2019

I bet Elon figures out how to add cars to the back of "Convoy Mode" when the trucks come out. S3XY Convoy (TM)

CharleyBC | 23. Januar 2019

If you draft so close that you are actually inside the back of the truck, your Wh/mile will approach zero.

draymond | 23. Januar 2019

Please ignore the advice from the OP. Drafting is ineffective unless you drive very close to the vehicle in front of you. For a semi that is around a car length away. Way too close for comfort. Obviously this is dangerous and it will annoy the truck driver. The OP, by using his "nimble Model 3" to dart into the fast lane and then slow down is also creating a hazard for the other drivers on the road. Just don't do it.

spuzzz123 | 23. Januar 2019

If he's using TACC it's well more than a car length away so why is this considered dangerous (M8B and draymond)? I agree with some other posters that this small amount of savings isn't worth sitting behind a semi for a long stretch, blocking my view and just making me feel claustrophobic. But I appreciate that someone took the time to actually measure it so I know now that its not worth it. I just don't understand the reaction here some times.

rwgallah | 23. Januar 2019

@SalisburySam - Thanks for the info.

@Magic 8 Ball - Yup :)

@draymond - To be clear, I'm not providing any "advice" on how other people should drive. Additionally, you are incorrect in your opinion that you need to be "around a car length away" to effective draft a truck. As far as danger, I performed this experiment is a safe environment and don't condone anyone doing any driving in an unsafe manner.

@spuzzz123 - you are correct that i was using TACC

@all - Chill. Please note that my testing was to provide data to the Tesla community not just my opinion.

adamwilt | 23. Januar 2019

Thanks, @rwgallah, very useful data. 11 Wh/mi is less than I would have expected. You have saved me from doing the experiment myself, and the paint on the front end of my Model 3 thanks you.

I often drafted 18-wheelers in my Honda Insight. Doing so kept me in lean-burn longer and gave me about 10-15% better efficiency (based on the realtime MPG readout in that car). But being "on the step" that way probably exaggerated the benefits; at a steady 65 mph on level ground in warm weather the Insight might see 65-70 mpg in lean-burn and drop to 55-60 mpg in rich-burn when it was purging the cat converter or needing just a bit more torque (ah, all these things I no longer need to worry about!).

But at manually-monitored 1.5 to 2 seconds back (e.g., settings 3 or 4 for TACC), it was a high-engagement, high-stress effort, and yes, one sees considerable paint erosion from rocks & gravel.

draymond | 24. Januar 2019

@rwgallah: It is not my opinion that effectively drafting another vehicle requires dangerous following distances. It is also not my opinion that swerving into the fast lane and then hitting your brakes is dangerous. These are facts. Your post was irresponsible.

jimglas | 24. Januar 2019

Good luck with our Darwin Award

rwgallah | 24. Januar 2019

@draymond - The facts are that I did not say "swerving into the fast lane" or "hitting your brakes". That is your spin of my words. Please read the original post without any pre-conceived opinions.

@all - i have not performed any drafting since i finished the testing because the disadvantages outweigh the advantages.

RES IPSA | 24. Januar 2019

I would hope that Level 5 autonomous cars will be programmed to draft off of one another at high speeds (like 5 feet off the bumper of the car in front you).

I guess it will just be unfortunate if you are the first car in the line

draymond | 24. Januar 2019

@rwgallah: You said "quickly pull out into the fast lane, slow down". I said "swerving into the fast lane and then hitting your brakes". My apologies for paraphrasing. Either way it's reckless and dangerous. Please stop doing this and please stop advising others to do it.

kcheng | 24. Januar 2019

A follow distance of 3, or 1.5secs is fairly innocuous and shouldn't bother a truck, but the draft won't be that strong. Now if you had truck in front and behind, that might be better! ;)

For the draft to be meaningful, you really have to be on a setting of 1.

Ultimately, the 4.7% improvement is barely better than putting your aero wheel covers back on. Yes, I know the OP had them on. I mean, if that amount of improvement is important to someone, then just put your aero covers back on.

It doesn't seem worth the additional risk of road damage to save 4 to 5%

Thanks for the research!

roberttown | 25. Januar 2019

Thank you for the information. The energy savings is too small for me to endure the lack of visibility.