Ever since the P85D initially hit the market and received a lower than expected EPA rating of 242 miles of range, there has been quite a bit of speculation on the 'how and why' behind the number, as well as what to expect with the other newly introduced "D" model - the standard 85D.
It turns out that a recently tweeted "sleep mode" for the dual motor cars will improve the P85D range to 253 miles, and will allow the 85D to achieve 270 miles of range - which is better than the 265 the current/original variants of the Model S (85, P85, P85+) are measured at. The software update is expected by the end of January.
Tesla Model S Family Ranges Updated
Tesla's Chief Technical Officer, JB Straubel explains:
"As the Model S family has expanded over time it has become more relevant to compare range from one variant to another with a consistent set of assumptions so our customers can know what to expect and make the best decision to fit their needs. This can be a bit difficult since the background test methodology and standards from the US EPA are evolving over time. There are also many customer vehicle configuration choices, both before and after purchase, that can affect range as much as or more than the vehicle platform choice itself. The most important example of this secondary configuration is the wheel and tire selection. This short paper will hopefully help to compare amongst all of the Model S family and understand how tire choices impact range as well."
Tesla also noted the "cruising" ranges on the Model S, something the company has used to promote the "D" family of cars previous to the official EPA ratings, as the AWD format allows for greater miles to be driven at speed.
Tesla Model S Family "Cruising" Ranges
Mr Straubel also gives consumers a primer aerodynamics, the effect tires and wheels have on range, as well as speed.
Check out the full report (and fancy graphs) from Tesla's CTO below:
The physics of aerodynamics affects all moving vehicles (gasoline or electric) the same: reducing efficiency and range at higher speeds. This effect is more pronounced at higher speeds since the drag force of the wind on the vehicle increases with the square of the velocity, from 35 to 70 mph it doesn’t just double but goes up by four times!
Despite this aerodynamic challenge, highway cruising is where the unique benefit of the dual motor cars, to torque sleep one of the drive units when not in use, is most apparent. Much like a modern computer that can actually sleep in between keystrokes, the dual motor Model S will quickly torque sleep a drive unit when torque is not needed and instantly wake it up as the accelerator is pressed to command more torque. It continues spinning while asleep and the digital torque wake up is so fast that the driver can’t perceive it. It is far superior to the slow and awkward engine start-up on stop-start hybrid vehicles.
The software update to implement torque sleep will be downloaded to the dual motor fleet by the end of January 2015 and will substantially improve the range of dual motor vehicles by roughly 10%. All tables and graphs in this paper are shown including the benefits of torque sleep.
Performance Tires and Wheels With tires and wheels there are some physics trade-offs between handling, traction and efficiency (rolling resistance and aerodynamic drag.) At the most intuitive level, as a tire becomes more sticky it will corner and accelerate better but also have modestly higher rolling resistance. Customers can make decisions on tires and wheels independently across all variants of the Model S. Of course these decisions are also often changed throughout the life of the car (summer vs. winter tires for example.)
The 19" Cyclone wheel and tire that we offer on most of our vehicles is the best configuration for range, efficiency and cost effectiveness. The 21" wheel/tire upgrade yields substantially improved handling performance and vehicle dynamics. The 21" performance tires are a more commonly selected option on our performance variants (P85, P85D and formerly P85+) since many of these performance-oriented customers want the best possible handling. There is however roughly a 3% reduction in EPA 5-cycle range (compared to the values presented in the table above) for the selection of 21" performance tires to any of the Model S variants. For customers who care about handling and performance driving this is a trade-off that we think is well worth it. For customers who are focused on range, efficiency and best value the 19" wheels can be configured onto any Model S variant to achieve the values in the EPA table above.
As an example calculation, if a standard Model S 85 with 19" tires having 265 miles of range is changed to 21" tires the range would be reduced by ~3% to about 257 miles. This is the range that most customers of the former P85+ configuration experience since nearly all of those variants are configured with 21" tires.
It is also worth noting that all new tires have a break in period for the first ~1,000 miles where the total vehicle efficiency is reduced by up to 5%. This can surprise (negatively) new owners or customers who have just changed their tires but will quickly improve back to a normal baseline.
Range vs. Speed There is naturally a strong sensitivity to range based on vehicle speed as mentioned above and we have discussed in some detail in the previous Model S range and efficiency blog.
The best way to see a more complete picture of this is actually in a graph of what range is possible versus driving speed.
And if we look in even more detail at the differences just between the 85kWh battery pack variants you can see the interesting complexity in how the dual motor operates. At some speeds the P85D is more efficient than the base 85 and equivalent to the 85D. At other, higher speeds the 85D and 85 are slightly more efficient, with higher range, than the P85D.
Summary Which battery, drivetrain and tire configuration is best for you will depend on what kind of driving you enjoy most. At Tesla we pride ourselves on transparency and providing customers clear data to understand our products. With the information above hopefully the choices and performance expectations are clear.
We have also added all of this information, and even more such as cabin heating and air conditioning loads (HVAC), day/night driving, windows up/down, 19" or 21" tires and outside temperature into a powerful range tool that you can use to simulate any driving conditions and any vehicle choices. Feel free to play with this and give us feedback if we can improve it!