Unexpected efficiency of 60kWh Model S?

[Saghost]

In a couple of threads recently people have been talking about the difference between the S60 and S85. As far as I can tell, the only difference is the battery pack, right? the same inverter, the same motor, and all the same aerodynamics?

The 60 kWh car is 4,407 pounds to the 85 kWh car's 4,630 according to this thread:

Just saying hello! New 60 on order. Late Jun 2014 delivery - Page 5

Given that, I'd expect the 5% lighter 60 kWh car to be a couple percent more efficient, mostly in the city. Also, I know that the 85 kWh cars currently include a tire upgrade that's said to extend range by 3%.

Here's the EPA data for the two - I'm not positive exactly how the test cars were configured:

Compare Side-by-Side

The 60 kWh car is around 7% more efficient, and more on the freeway than the city according to the EPA. Not what I was expecting at all.

So two questions: First, does real world experience match the EPA data - is the 60 kWh car using almost 10% less electricity to cover similar drives?

Second, does anyone have an explanation for why the 60 kWh car is more efficient? It seems to be a much larger factor than the weight difference should produce.
Walter

 

[Jeff Miller]

In a couple of threads recently people have been talking about the difference between the S60 and S85. As far as I can tell, the only difference is the battery pack, right? the same inverter, the same motor, and all the same aerodynamics?

The 60 kWh car is 4,407 pounds to the 85 kWh car's 4,630 according to this thread:

Just saying hello! New 60 on order. Late Jun 2014 delivery - Page 5

Given that, I'd expect the 5% lighter 60 kWh car to be a couple percent more efficient, mostly in the city. Also, I know that the 85 kWh cars currently include a tire upgrade that's said to extend range by 3%.

Here's the EPA data for the two - I'm not positive exactly how the test cars were configured:

Compare Side-by-Side

The 60 kWh car is around 7% more efficient, and more on the freeway than the city according to the EPA. Not what I was expecting at all.

So two questions: First, does real world experience match the EPA data - is the 60 kWh car using almost 10% less electricity to cover similar drives?

Second, does anyone have an explanation for why the 60 kWh car is more efficient? It seems to be a much larger factor than the weight difference should produce.
Walter

1. It's hard to answer this question definitively because there may be correlations between the car one chooses to buy and how aggressively one drives. That said, if you take the data at face value, the self reported average wh/m of the 60s are about 6% less than the 85s. See tables 4 and 5 and figure 6 here:

Lifetime Average Wh/mi - Page 96

On the other hand, the three best wh/m averages (~260) are all from people driving 85s.

2. I am puzzled by this as well. I agree that a 5% difference in weight is not enough to account for a 6 of 7% difference in efficiency.

 

[jerry33]

Second, does anyone have an explanation for why the 60 kWh car is more efficient? It seems to be a much larger factor than the weight difference should produce.

I suspect it has to do with the EPA test and the fudge factor that they use.

 

[Mnlevin]

I dont know how accurate that really is. I do think it is more to do with the driving habits and daily use. We have had the car for just over 1 year with 24k miles. Our Wh/Mi is 311, which i dont think it high or low. Mostly highway miles. somedays it is running 300 @ 70mph and some days it is 330-340. But the life time over the 24k may be more telling and probably close to the average 85 as well. We dont drive it hard but not easy either.

 

[Saghost]

1. It's hard to answer this question definitively because there may be correlations between the car one chooses to buy and how aggressively one drives. That said, if you take the data at face value, the self reported average wh/m of the 60s are about 6% less than the 85s. See tables 4 and 5 and figure 6 here:

Lifetime Average Wh/mi - Page 96

On the other hand, the three best wh/m averages (~260) are all from people driving 85s.

Wow. Someone spent quite a while assembling statistics there.

As I read it, it does appear to support the 60 being more efficient - the bottom three may be 85s, but half of the lower twenty are 60s (and 40s, since that's a software limited 60,) despite being only a quarter of the sample. By contrast, only two show up in the top twenty. It doesn't really seem to be showing as much benefit to the 60 kWh cars as the EPA data suggests, though.

Clearly the biggest take away is that other factors matter more - the temperature, terrain, and technique that we talk about for all EVs.
Walter

 

[Jeff Miller]

As I read it, it does appear to support the 60 being more efficient - the bottom three may be 85s, but half of the lower twenty are 60s (and 40s, since that's a software limited 60,) despite being only a quarter of the sample. By contrast, only two show up in the top twenty. It doesn't really seem to be showing as much benefit to the 60 kWh cars as the EPA data suggests, though.

Clearly the biggest take away is that other factors matter more - the temperature, terrain, and technique that we talk about for all EVs.
Walter

I think I agree with that.

One thing that is interesting is that because the Model S is so heavy and so aerodynamic, rolling resistance accounts for a much bigger fraction of the total resistance than it does for your average car. At high speeds (75 mph) it equals half the wind drag, and at lower speeds it's most of the resistance:

Regen vs. Coasting - Page 4

Given the greater relative importance of rolling resistance, together with whatever resistive losses are incurred from going up and down hills, I wouldn't be completely surprised if the 5% difference in weight corresponded to a 3% difference in efficiency.

 

[Sparrow]

I own a P85 with 19" wheels and while my car is now in the shop I have a 60 with 19" wheels loaner. Admittedly I haven't driven alot of miles in the 60 (150 miles), but so far I can not see any real difference in the efficiency of my P85 verses the 60 loaner.

 

[arg]

So two questions: First, does real world experience match the EPA data - is the 60 kWh car using almost 10% less electricity to cover similar drives?

It has been speculated previously that the EPA tested the 60 with 19" wheels/tyres, and the 85 with 21", which would account for a big chunk of the difference.

 

[jerry33]

On the other hand, the three best wh/m averages (~260) are all from people driving 85s.

That is probably due to trip length. I'd guess that 85 folks have a longer commute than 60 folks. Even in an electric car it takes time for the tires and reduction gear fluid to warm up and be at their most efficient. I'd wager that the amount of friction cold reduction gear fluid has will be be surprisingly large.

In the Prius PSD Aragorn labs measured a 25% difference in friction between cold and hot fluid [from memory]. Some Prius owners attach an electric heater to warm the fluid in winter. The reduction gear in the Model S won't have the same amount of friction, but it won't be zero either.

 

[hikerockies]

I own an S60 and an S85. My wife and I both work from home, so both cars are driven primarily to/from kids' schools and errands around town, and we both drive whichever car we feel like (no his and her car). With similar driving pattern and style, I am noticing that it is harder to keep S85 under 300 Wh/mile. On days with 65-75 deg temp, S60 quickly settles around 280 Wh/mile while S85 hovers around 305 Wh/mile. S60 has Goodyear tires while S85 has LRR Michelin tires. I have had S85 for just couple of weeks, so I am waiting to see more long term numbers.

 

[wycolo]

EPA probably did not conduct its testing under identical ambient conditions, so did they trust a correctional factor to achieve this?
--

 

[Saghost]

I own an S60 and an S85. My wife and I both work from home, so both cars are driven primarily to/from kids' schools and errands around town, and we both drive whichever car we feel like (no his and her car). With similar driving pattern and style, I am noticing that it is harder to keep S85 under 300 Wh/mile. On days with 65-75 deg temp, S60 quickly settles around 280 Wh/mile while S85 hovers around 305 Wh/mile. S60 has Goodyear tires while S85 has LRR Michelin tires. I have had S85 for just couple of weeks, so I am waiting to see more long term numbers.

Huh. So you are seeing almost ten percent better with the S60 in what should be an even comparison, despite the S85 having what are supposed to be better tires. That's very interesting.

We are sure the S60 uses the same motor and inverter, right? (I think I read someone got Tesla to upgrade them by replacing just the battery pack for $20k or so in one of these threads...)
Walter

 

[Dave EV]

It has been speculated previously that the EPA tested the 60 with 19" wheels/tyres, and the 85 with 21", which would account for a big chunk of the difference.

That is my best guess as to what's going on here.

I have had S85 for just couple of weeks, so I am waiting to see more long term numbers.

It will take at least a thousand miles for the car to break in.

EPA probably did not conduct its testing under identical ambient conditions, so did they trust a correctional factor to achieve this?

The manufacturer typically submits test results to the EPA, the EPA does not necessarily test the car.

 

[wamochi]

I own an S60 and an S85. My wife and I both work from home, so both cars are driven primarily to/from kids' schools and errands around town, and we both drive whichever car we feel like (no his and her car). With similar driving pattern and style, I am noticing that it is harder to keep S85 under 300 Wh/mile. On days with 65-75 deg temp, S60 quickly settles around 280 Wh/mile while S85 hovers around 305 Wh/mile. S60 has Goodyear tires while S85 has LRR Michelin tires. I have had S85 for just couple of weeks, so I am waiting to see more long term numbers.

that's very interesting - are they both on 19s ?

- - - Updated - - -

It has been speculated previously that the EPA tested the 60 with 19" wheels/tyres, and the 85 with 21", which would account for a big chunk of the difference.

our 60 is arriving 4 weeks from today with 21's. will be interesting how it stacks up to the stats.

 

[jerry33]

Huh. So you are seeing almost ten percent better with the S60 in what should be an even comparison, despite the S85 having what are supposed to be better tires. That's very interesting.

So far, I have not been able to get the Michelins as low as the Goodyears on my commute. The best I've done is about 15 Wh/mi worse. Now the Michelins are better in that they don't skip on corners but they don't seem to be as energy efficient. However, it's kind of scary to have the tires start to skip on a low speed corner, so the small amount of extra energy consumption is worth it. To put numbers on it, going in the "good" direction the GYs did as well as 193 Wh/mi and were seldom over 200. Best so far with the Michelin is 205 and I've only achieved that once. 210 to 220 is more typical.

Note: At highway speeds the Michelins may be better for energy consumption. I don't really have the ability to tell because I don't have a regular enough highway drive to get a consistent number. My suspicion is that Tesla got the 3% lower number from driving around their test track at 65 mph or so.

 

[hikerockies]

that's very interesting - are they both on 19s ?

Yes.

 

[iKhalid]

I'm not a "spirited" driver but I'm not your ideal driver either. I floor it a few times and drive above speed limits and speed up at corners to test things... It's still a new car to me after all (4,800KM).

I started at 214wh/km (344wh/mi) and now it's decreasing; my average is 207wh/km (333km/mi). I also noticed that my last 50KM stats rarely goes more than the EPA rated line (200wh/km).

On my roundtrip to Montreal from Ottawa (~200km), I was maintaining 110km/h using the cruise control with the AC on, and I had "almost" a flat line around 200wh/km on top of the EPA rated line. So my average was ~200wh/km. The consumption vs distance had only 10km difference in that say 210km travel distance.

Looking at the reported numbers from various owners, I think the S60 is more efficient. I had the same concern in a different thread but I never got a definitive answer.

 

[Sparrow]

Now at 310 miles and 232 wh/m with the loaner 60 with 19" turbine wheels, Michelin tires and total mileage on the car is over 2600 miles now. This month I have 830 miles and 234 wh/m on my P85 with 19" wheels and Goodyear tires.

 

[Cal1]

I'm considering a MS60 as a second ev. Our 85 typically charges to 230 on a daily charge, what are you guys with 60 and 40kwatt battery's getting for a daily charge around town? Could not find this anywhere in my searches. Everything was on the max trip charge.

 

[youlikeadajuice]

Since 5.9, I've been seeing 182 rated miles on a 90% charge. For the thread, I'm getting 307 Wh/m lifetime after 2 winters and 28,000 miles. I'm not a super conservative driver, but not super aggressive either. I also have hills to deal with on my daily commute.

I'm considering a MS60 as a second ev. Our 85 typically charges to 230 on a daily charge, what are you guys with 60 and 40kwatt battery's getting for a daily charge around town? Could not find this anywhere in my searches. Everything was on the max trip charge.