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The quoted 31mi/hr is in ideal miles, not rated miles. There are many other threads about this.

1 hour at 240/40A gives you 9600Wh.
9600Wh / (85000Wh/300miles) = 33,9 miles (ideal)
9600Wh / (85000Wh/265miles) = 29,9 miles (EPA rated)

Given that it charges at ~80% of available amperage (I believe, I'm not saying it's accurate), it turns 29,9 miles into 23,9.
If the charger is ~90% efficient, you end up at 22.

@glaserud - the 80% factor has already been taken into account at 40A rather than 50A. However, the charging efficiency seems to be around 86-87% (energy from the wall to energy stored in the battery), and the rate of charge varies depending on the battery's current state of charge (it is slower when nearly empty or nearly full) and temperature.

Personally, I usually see around 23-24 rated mi/hr, but it varies somewhat. What is the voltage the car shows while charging?

It generally charges more quickly than it shows. It might say 21 miles, but in fact it's charging more.

I have only had my car for a week or so and I noticed the same slower than expected charge rate (around 24mi/hr). But most days during the week I was only using about 20 miles of range, so my battery was already close to a standard charge. I took a longer trip over the weekend so that my battery was about half empty, then I noticed I was getting about a 31mi/h charge rate. It seems to be kind of like a balloon where it gets harder/slower to fill up when it gets closer to the limit.

If my memory serves me correctly: Standard LiIon charging profile is to apply a steady current until top cell voltage is reached. Then you decrease the current to maintain the voltage until you're done charging.

So yes, the balloon analogy is about right. :)

As diego sez, watch the actual total rather than the rate number. There's a bug that under-reports the rate.

@glaserud - I don't know if this applies to the chemistry used in the Model S, but li-poly batteries charge slowly when they are almost fully depleted, then after a bit of charge they stay constant until the constant voltage phase when it the charge rate drops off again. The Model S may actually prevent you from discharging the battery that low, so it might be linear all the way down to what the car calls "zero charge".

Have you checked which voltage you actually get while charging? It usually isn't the nominal 240V, because of cable losses in your house wiring. Not unusal to see 220 - 230 V. The charging screen shows the actual voltage received by the car. This reduces the miles per hour in proportion to the voltage loss.

A few things to consider:

* 40A rated circuit provides 32A continuous for charging.

* 240 VAC nominal can end up at 220 or less at the charger

* actual charging efficiency averages about 85% (92% max at optimum temperature, full empty...can be less than 85% when nearing full, cold, etc.)

combining the 2 results in charge rate of 21 "rated" mph (308 Wh/mi)...well within expected (if your circuit is 40A rated - if you have 50A rated circuit, then the charge will be up to 40A)

Spreadsheets/tables at http://EVTripPlanner.com/calcs.php

As others have mentioned, variation in actual voltage may be the difference. I also get 22-24mi/hr of charging with 14-50, but the voltage coming in to my house is only 204-210v. I didn't know that until my electrician installed 14-50.

I had my electrician install #6 wire and change the breaker to 50Amp so I can pull 40Amp charge. The house has 200Amp circuit.

I find I get about 26 to 29 MPH @ 40 Amps / 240 volts.