Aero Efficiency

[FloridaJohn]

Thanks Zoomit. So you're model suggests between 9-10.4% range loss. Since we've seen reports of -4% in (preliminary) real world tests, I guess the real range loss should be between 4 to 4.7% @85-55mph. A .7 percent point increase. Like you wrote: 'very minor'.
Every little bit helps, I guess.

It looks like he is comparing 19" Sport wheels to 18" Aero wheels with hubcaps installed. The preliminary 4% test was only comparing the 18" wheels with and without the Aero hubcaps installed.

We haven't seen any real world tests comparing 19" Sport wheels to 18" Aero wheels with hubcaps installed. I would guess the improvement would be greater than the hubcaps installed vs not installed. All indications seem to predict it will be around 10%, but we won't know for sure until someone runs the test.

 

[suwaneedad]

I built a set of simple models based on the three dyno coefficients that Tesla provided to the EPA and shown in post #33 above. These models are approximate and should be considered skeptically until validated with empirical data, but show a minor trend I didn't expect.

The 19" wheel/tire combination appears to have an increasingly detrimental effect at lower speeds. This trend is not obvious looking at the second graph just showing Range Lost and Consumption Increase, but is obvious when the percentages are plotted in the third graph.

If this is trend is real, I suspect it is caused by the higher rolling resistance of the 19" tires, which appears to be more dominant at slower speeds. These models assume constant speeds, no acceleration/deceleration, so the effects from the different wheel/tire rotational inertias isn't a factor. In total, this trend is a very minor, being only a few percentage points different over this range of speeds.

Again, I wouldn't focus on the specific values. They could easily be off, but I think the big trends are real.

Thanks a million @Zoomit. I'm happy that you found a new discovery in building these, so that your work wasn't purely for my benefit! What I've learned here, since I will frequently use M3 for 500mi trips, is that (focusing on the 250mi horizontal on the first chart), I can travel 250mi at 84mph on the 18s, and 250mi at 78mph on the 19s. For my 500mi trip, that translates to a 24 minute increase in drive time if I elect the 19s.

What's also interesting for me is to see that the range delta is so tightly in the ~9-10% range on the highway, essentially regardless of speed. Good stuff. Thanks again.

 

[SageBrush]

see that the range delta is so tightly in the ~9-10% range on the highway, essentially regardless of speed

nah

 

[sreams]

It seems like this should not be correct. Aero has a greater effect as speed increases. The graph does not reflect this.

 

[DarthPierce]

It seems like this should not be correct. Aero has a greater effect as speed increases. The graph does not reflect this.

That's because it's not *just* aero differences. It's also that the 18 inch michelin tires themselves have lower rolling resistance (at all speeds)

 

[Zoomit]

It seems like this should not be correct. Aero has a greater effect as speed increases. The graph does not reflect this.

Aero does have a greater effect with speed and that's shown in the second graph with the green line. Consumption increases for the 19s faster than it does for the 18s as speed increases.

I've reviewed the spreadsheet and can't find any errors. The calculations are relatively straightforward compared to my "home grown" efficiency and battery models. The calculations are:

1. Add EPA coefficients to equation: Road Load Lbf = A+B*V+C*V^2, where V is speed
2. Convert Road Load Lbf to kW at all speeds
3. Bias the road load up by 8.6% to account for internal losses (ratio determine by calibrating to EPA range data)
4. Calculate Wh/mi and range over the speeds
5. Calculate differences and percentages
6. Plot

 

[Zoomit]

Thanks a million @Zoomit. I'm happy that you found a new discovery in building these, so that your work wasn't purely for my benefit! What I've learned here, since I will frequently use M3 for 500mi trips, is that (focusing on the 250mi horizontal on the first chart), I can travel 250mi at 84mph on the 18s, and 250mi at 78mph on the 19s. For my 500mi trip, that translates to a 24 minute increase in drive time if I elect the 19s.

What's also interesting for me is to see that the range delta is so tightly in the ~9-10% range on the highway, essentially regardless of speed. Good stuff. Thanks again.

You're welcome!

This is diverging off topic, but your simplified 500 mi trip doesn't need to be 24min slower with the 19s. If you traveled at the same speed in both cars (84mph), the 19s would use 15.7kWh more electricity. If we assume a Supercharging power of 100kW, that extra 15.7kWh used would take just under 10 additional minutes to recharge. At California rates($0.20/kWh), this is also an extra $3.14. At Georgia rates ($0.16/min above 60kW), this is an extra $1.50.

 

[suwaneedad]

You're welcome!

This is diverging off topic, but your simplified 500 mi trip doesn't need to be 24min slower with the 19s. If you traveled at the same speed in both cars (84mph), the 19s would use 15.7kWh more electricity. If we assume a Supercharging power of 100kW, that extra 15.7kWh used would take just under 10 additional minutes to recharge. At California rates($0.20/kWh), this is also an extra $3.14. At Georgia rates ($0.16/min above 60kW), this is an extra $1.50.

Great point. I went to ABRP yesterday, hoping to pop in my typical journey in both 18s and in 19s, but ABRP currently only supports Model 3 LR. To your point about the real impact being charging time, I was thinking that a refined ABRP tool would be able to plot that delta very explicitly for me (given that charging rates decline above 50%, and so on).

Regardless, I am very much used to varying my speed in order to obtain my range goals, given my 6.1 years tooling around ATL in my Leaf. So to see that your charts predict very little speed difference is needed (~6mph) to negate the range impact of 18s v 19s when at highway speeds ~80mph, and now that you're also quantifying the time delta further given some supercharging assumptions, I've come back fairly confident that I will be going with the (much prettier to my eye) 19s. I also drove next to a compact car yesterday with hubcaps, which were just a bit wonky, and thought "damn I do not want my Tesla with even slightly out of round hubcaps someday, and I'm surprised Tesla designers concluded that was an OK thing either for the brand." I'm also an empty nester now, no longer racing to get to my road trip destinations due to managing too many little bladders in the back seat. It's a glorious thing, having time on my side.

 

[silentsnow31802]

Im seeing about a 4% difference between aero covers and no aero covers over the span of 1800 miles so far. Have not quite hit the end of my testing yet though.

I have finished my test. I did 3700 miles with aero covers on and 3700 miles with no covers but center caps and lug nut caps installed. I got 290wh/mi with no covers and 280wh/mi with covers. My test was just driving back and fourth from work and home, same times of the day every day. There were some small trips to the store in between there but not enough to make a difference at the mileage i covered.

 

[omgwtfbyobbq]

I have finished my test. I did 3700 miles with aero covers on and 3700 miles with no covers but center caps and lug nut caps installed. I got 290wh/mi with no covers and 280wh/mi with covers. My test was just driving back and fourth from work and home, same times of the day every day. There were some small trips to the store in between there but not enough to make a difference at the mileage i covered.

Do you know what you're average speed is?

 

[silentsnow31802]

Do you know what you're average speed is?

I would have to guess about 70 mph.

 

[omgwtfbyobbq]

That's pretty fast for an average. Are you able to cruise at 90+mph for a significant portion of your commute?

 

[Zoomit]

I would have to guess about 70 mph.

Yeah--that's a super fast average for 3700 miles. If you did 50% of it on the highway at 75 mph, the average might be 50 mph.

Nevertheless, the data is very helpful. About 3.5% difference over "typical" driving. (not just high speed highway)

 

[YauKwan]

Would we get even better efficiency taping up the holes in the aero cover?

 

[T34ME]

Would we get even better efficiency taping up the holes in the aero cover?

Not according to Tesla engineers. The wheel covers are optimized the way they are. Why reinvent the wheel?

 

[SageBrush]

Would we get even better efficiency taping up the holes in the aero cover?

You need rotor and pad ventilation or you risk poor performance from over heating with brake use.
It is not an issue if regen is mostly used but Tesla has to consider unusual routes and dumb drivers.

We are all victims of the least common denominator.

 

[silentsnow31802]

Yeah--that's a super fast average for 3700 miles. If you did 50% of it on the highway at 75 mph, the average might be 50 mph.

Nevertheless, the data is very helpful. About 3.5% difference over "typical" driving. (not just high speed highway)

My commute is 72 miles one way. I go home at 2AM and do a minimum of 80 the entire way. I work and live less than a mile from the highway. On my way to work around 1:30PM there is minimal traffic and the carpool lane/express lanes are usually empty. I can keep a pretty good pace.

 

[Young]

Temperature change can be a factor, too. We are having unusually warm weather recently.

 

[Zoomit]

Ok, that’s definitely a lot of high speed driving!

Seasonal temperature swings can definitely affect the results, as @Young said.

 

[silentsnow31802]

Ok, that’s definitely a lot of high speed driving!

Yes it is lol. I don't like spending all my time commuting. That also explains my high energy consumption while every one else is getting in the 250wh/mi range. I get an average of low 400's doing the same commute in my S.