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This is the exact question I've had as well, I wish I knew the calculation to determine.

Look at the Roadster chart at

http://www.teslamotors.com/blog/roadster-efficiency-and-range

Model S CdA is approx same as Roadsters, so aerodynamic drag is same Roadster is about as aerodynamic as a brick, Model S is larger (A) but much better in Cd. However rolling resistance ("tires" in the chart) is higher because it is heavier car. That's pretty constant at any speed.

Roadster uses about 230Wh/mile at 55mph, we know that Model S 85kWh battery should last 300 miles at 55mph. That gives us 283Wh/mile.

Difference is 53Wh. Roadster uses about 330Wh/mile at 75mph, so Model S should use 383Wh/mile at 75mph. 85kWh/383Wh/mile = 222 miles.

Estimation, but should not be far from truth.

I can't be sure, so maybe someone who knows about this will chime in. But I think they listed "55" to show the average between city and highway driving. Maybe when it's closer to delivery there will be more specific information about different speeds. I know there's a good deal of detail on the Nissan Leaf page about what causes different levels of strain on the battery (heater on/off, speed, incline, etc.,). I would hope Tesla will provide this level of detail as well, but just needs more time to gather the data.

@Timo,

Awesome! I always estimated a 25% penalty for realistic highway driving. Virtually all my driving is highway. This fact should weigh heavily on the battery size decision for anybody doing a lot of highway driving. Looks like your math is almost dead on with my guestimate.

Thats an excellent question! You know when computers first came out I always wanted the biggest memory, etc.. but realized it is unnecessary if you do not use all the computing power.

Its the same with the battery but the unknown of highway distance, battery degradation over time and cold weather effects on distance makes me weary to not get the 300 mile battery. I would save $10,000.00 if I really do not need the 300 mile pack(going with 230) but the comfort level would be much higher with the 300.

I did some calculations and it matches Timo's value (75mph=225miles)

http://www.teslamotorsclub.com/showthread.php/7072-Highway-Range-Ignoran...

http://www.teslamotorsclub.com/attachment.php?attachmentid=3739&d=1324526972

Me, you, and Sammy Hagar can’t drive 55 so I came up with a formula of my own that works really well for me, I hope it’s not too complicated to be useful to others. Most people drive over the limit by 4 mph, 9 mph or 14. In my calculation 4 mph, 9 mph, and 14 mph correspond to 5, 4, and 3.

Tesla lists “Rated Range” which I call affectionately call ‘Overrated Range’. They also list distance on the Tesla Google maps in your car. The difference between these numbers gives me a really good range/speed assessment. Multiply the difference by 5, 4, and 3. The answer that is closest to the ‘overrated range’ dictates what speed you can go. (14, 9, 4 mph over the limit)

For example:

If you plan to drive 100 miles and your ‘overrated range’ is 150, what speed can you drive? The difference is 150-100=50. 50 x 3=150, therefore you can drive 14 mph over the limit and make it.

In fact, you could drive 120 miles at only 4 mph over the limit. 150-120=30. 30 x 5=150.

If you’re taking a trip and stopped at a Supercharger. How much ‘overrated range’ do you need in order for you to make it to the next Supercharger if it is 180 miles away and you want to drive 9mph over the limit? A:270, because 270-180=90. 90 x 4=270.

If I am driving and my ‘overrated range’ has fallen, I immediately know how much to lower my speed so I can still make it to my destination. This allows for adjustment to ‘black swan’ factors like head wind or elevation while you drive.

What this formula boils down to is that I lose 20% of ‘overrated range’ at 4 mph over the limit. 25% at 9 mph and 33% of ‘overrated range’ at 14 mph over the limit.

You state you need 270 rated miles to drive 180 miles at 64mph?

I think your formulas are a bit off. I can drive my P85D over 200 miles at 80mph with a 100% charge with 256 rated miles.

Uh, math alert: The speed limits for freeways varies from 55mph to 80mph.

Pray tell, how does a formula of (speed limit)+x yield any actionable information. Now, if you are using 55 as the speed limit, perhaps your method works.

Driving 55mph on an Interstate will get you dirty looks or worse.

Yea, none of that made sense, especially 90x4=270.

There is no blanket formula to figure this out. 70 mph here in New England is going to use up more miles than 70 mph in FL.

If your avg energy usage it at or below 295 kwh, you will get Rated miles or better.

http://my.teslamotors.com/forum/forums/bridged-missouri-gap-s85d

Distance depends on many variables. Tesla model, wind, temperature, load, road traveled, etc.

See the link above for real experience numbers for crossing Missouri.

@ jordanrichard | July 9, 2015

If your avg energy usage it at or below 295 kwh, you will get Rated miles or better.After driving my 85D a few weeks ago, I noticed that the projected range was exactly the same as my rated range. I checked the numbers and concluded that it reported that I was using 290 Wh/mi (average of most recent 30 minutes). So I thought that meant that the magic factor defining what Tesla means by "rated range" was 'range if you use 290 Wh/mi'. Is this correct?