Ok, I looked at the Autodata spreadsheet. Still not certain what their methodology is for collecting the data, but I did realize that one key difference between their estimate and our VIN based data is that they are reporting only on the U.S. market, while our data includes Canadian sales (and eventually global sales).
Setting the results for May aside, they show 7,506 U.S. sales so far this year. If I assume 1,700 deliveries in May, and 1,450 in April (I subtracted 100 loaners from May), that is ~3,150 deliveries so far in Q2. Adding the additional ~4,900 in Q1 you get ~8,050 North American deliveries in 2013, with ~544 being Canadian.
The Canadian number is maybe a bit high as a percentage of the market, but Canadian deliveries did not even begin until Christmas 2012, so the vast majority have been delivered in 2013. Looking at the reservation tally, there were ~877 Canadian reservations as of February 1st (including the Sigs), and some small portion (maybe 50-100) were delivered at the end of December. Once you factor in cancellations, 544 Canadian deliveries so far in 2013 looks very credible.
Coming back to the May Autodata report, we know it's U.S. only. If we assume 1,700 total deliveries, and Autodata is only seeing 1,425 U.S. deliveries, that could mean that Tesla just did a big batch (275) for the Canadians, or it could mean a somewhat smaller batch for the Canadians (175), and maybe a somewhat smaller (but still credible) delivery number of ~1,600.
Bottom line is that I am not in a position to question the Autodata numbers, and for 2013 as a whole they are extremely credible.
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I should also point out that if Tesla did a batch of Canadian cars, it would slow the delivery flow compared to the trend (because every Canadian car is by definition far from the plant in Freemont). So a big batch of Canadian cars would look like a slow sales month, almost by definition. If so, you would see that self correct in the following month as deliveries snapped back to trend.
I'm not certain of all the differences between Canadian and U.S. cars, but even minor differences are probably enough to make Tesla prefer big batches over small ones, just so that the work flow is not disrupted as much.
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One more thing, the Autodata numbers for Tesla are an "estimate". Take that for what it is. Here is the spreadsheet for anyone interested -
http://www.motorintelligence.com/fileopen.asp?File=SR_Sales61.xls