SuperCharger Taper Rates and Quoted mi/hr

Miles per hour is averaged over the whole session if I remember right. Since the car charges much faster in the lower part of the battery, you'll see a higher miles per hour at every point in the session if it starts from a lower SoC. You should really be paying attention to kW/kWh rather than mph.
Walter

Not to be rude, but you think your buddy driving your car is the "EV greenhorn"? I think there are two EV greenhorns here. First thing you need to do is read the page about supercharging on the Tesla web site:
Supercharger | Tesla Motors
if you want to know what all this "taper fuss" is about.
Then ask your friend who is driving what his state of charge is when he starts supercharging and what kW he is drawing, and if there is another car charging from the paired supercharger. As Saghost said the miles/hr displayed is an average over the session up to that time and is neither the starting rate, the rate at that instant, or the ending rate.

Evbwcaer said:

At 230 miles of rated range the power going in has been as high as 230mi/hr and as low as about 180mi/hr. Similar temps, no other users, same rated range in the battery, all at Superchargers showing they are capable of well over 300mi/hr.

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No no no.

You're apparently missing what we are trying to tell you about the way the car reports miles per hour of charging. What you actually saw is that "at 230 miles of rated range, the average power going into the car over the entire session to that point was as high as 230 miles per hour and as low as about 180 miles per hour."

The car reports instantaneous kW, aggregate kWh, and session average mph. The session average miles per hour will vary at any given point in the session based on where you started the charge and how much really fast charging you managed before the taper hit. The average session miles per hour at any given instant is meaningless in assessing your momentary rate of charge.
Walter

Evb,

Last month I took a road trip from California to Milwaukee, Minneapolis and back, primarily using Superchargers. I believe I Supercharged 37 different times. I changed units from distance to energy so that the display on the touchscreen indicated kWh instead of MPH. At Lovelock I was second fiddle, so received a reduced rate. At Goodland one morning, my initial rate was 53 kWh with a 30% share of charge remaining in the battery. At Glenwood Springs the initial rate was around 70% (at 45% SOC) initially, perhaps due to the fact that it was early in the morning, and the temperature was around 40 degrees.

All my other charging stops with battery levels varying between 5% and 35% started out anywhere between 113 kWh to 118 kWh, and began to taper around 40-50% (I really did not stay in the car to monitor.)

I suggest that your friend change the charging units to energy and watch the touchscreen after plugging in. In my cases, the current would ramp up quickly to 114-116 kWh, then fluctuate a little for a few seconds and finally increase to 117 or 118 for some time before starting to taper. A couple of spots never reached that high, but for all intents and purposes a 2-4 kWh reduction is immaterial for adding 40-80% capacity.

Evbwcaer said:

Thanks guys, I see what I am missing with the average vs. the instantaneous mi/hr interpretation. I had been figuring the kw by doing the math. Is straight kw the measure to go by, or does it matter regarding charge rate how much comes from amperage vs voltage?

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Use kW, derived from Volts * Amps / 1,000.

To get instantaneous rate mph charge rate, good round numbers on an 85 are 3.33 (10/3) rated miles per kW for DC charging and 3.0 rated miles per kW for AC charging; the difference is due to AC->DC conversion losses.