My car is an S85 Cyclone wheels and had about 10lbs of "stuff" in the car at the time I measured the weight. My car was manufactured in the 2nd week of March 2014, it has the Ti plate from the factory. I'd imagine it also has the "hundreds of pounds" shaved off the way Elon described in the Q2 call, although my number did not come in at much below what is spec'd on the website. The Teslamotors website claims : 4647.3lbs My car came in at exactly 4600Lbs.

When doing this kind of measurement you always have to consider the tolerance of the measuring instrument, that is to say the scale. Do you know this parameter?

I do not know what exactly they specified, but I can tell you this. I drove up to the scale the 1st time, and the car weighed 4610. I drove away. Then I remembered I had my work bag in the car, I turned around drove back on the scale and removed the work bag, the car came in at 4600lbs. The bag weighs exactly 10lbs. Based on that I'd imagine the scale is pretty accurate and repeatable.

If that scale is 1% off it would be 0,1 lbs for the bag, but 46 lbs for the car.... If the scale is used for business purposes you can assume it's closer to 0.1% margin of error.

If the accuracy of your scale was 1% you would have a tolerance of 46 lbs. Then 4600 + 46 = 4646 lbs is very close to the value given by Tesla (4647.3 lbs). I suppose that Tesla made the measurement with a much more accurate scale (accuracy = 0.01%). In fact Tesla managed to give a value of the weight precise to the first decimal (.3). So I think that the value that you measured could be very close to the value measured by Tesla depending upon the accuracy of your scale.

That shows the scale is precise but not necessarily accurate. Precise means repeatable results. Accurate means the total weight measured is close to the actual value. However, even if it was able to show the 10lb difference, it does not eliminate the possibility of a bias in the scale (it could for example be biased low, so the difference in weight may be accurate, but the total weight number may not be). That being said, it still gives a figure that does not show a few hundred pound difference from the website claim.

Actually on TMC the electrons useful to charge the Model S have been counted and weighted! There is a thread on this matter on TMC. Of course such a measurement is negligible.

come on, if you PAY for the electrons at home they must weight more than the FREE ones at superchargers...

Actually for fun I was going to mention that the battery was at 74% charge, then actually calculate out the mass of the electrons, but I figured that would be to nerdy (as much fun as it would have been to revisit college G-chem) so here's the thread where some guys did do the math. How much more does a fully charged MS weigh?

Where does one go to get a car weighed? I'd like to get my 60 weighed to try to settle that ballast-or-no-ballast debate once and for all; according to the owner's manual, the 60 should be 223 lbs lighter than the 85.

If you didn't pay for a "certified" scale measure, its not valid. Any old weight scale at a wholesale landscaping/rock supply outfit will give you a weight, but its a relative weight. They also state not to use the weight as something valid, it works for their use but not yours. Meaning they weigh something without something, then with something, and and the difference in weight is the price you pay for the "stuff". Its great if you don't care there's 50-200lbs of error in the initial measure that essentially creates a "zero" for their purposes. The landscape shop close by where I live said I can use their scale to weigh my truck camper/rig before I do my long haul camping trips for free. That works for me and adds to my own, as well as other's, personal safety on the road. I weigh the rig unloaded, then loaded, and the difference is my true known loaded weight. Therefore I know what my hauling load is and how the truck will handle and brake with the "known" amount. I also know if I'm over what I built my rig's hauling capacity to be (rims, tires, suspension, brakes, etc.). A certified scale is calibrated and checked by the state's weights and measurements department regularly to ensure accuracy or else there's a stiff fine, hence why they charge you a pretty steep rate. Things hang/bind that shave or give false lbs. reading to the scale when not checked regularly. Tesla I'm sure paid for a certified scale measurement I trust Tesla's scale over yours, sorry.