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The Software used to design and engineer the Model S....

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And Autocad
http://investors.autodesk.com/phoenix.zhtml?c=117861&p=irol-newsArticle&ID=1377411&highlight=

...Tesla Motors. Enerdel is an advanced lithium-ion battery maker currently working with the U.S. Army and other major contractors. Tesla is a provider of high-performance electric vehicles. Enerdel will utilize Autodesk software solutions to create a Digital Prototyping process for the development of its next-generation lithium-ion batteries, while Tesla Motors will use Autodesk's 2D & 3D design software to design and style all-electric vehicles.
"Tesla Motors has been using Autodesk Alias to optimize and visualize our vehicle designs for some time," said Franz von Holzhausen, chief designer. "We look forward to expanding our partnership with Autodesk to rapidly accelerate our innovation, performance and time to market as we expand our all-electric portfolio."
And Solidworks

http://www.jobsearchusa.org/job.php?id=100445418

Qualifications:
At least 5 years recent experience working in CAD
At least 5 years recent experience working with GD&T
Experience with assemblies, multi-model links, design in context
Experience with detailed drafting and modeling
Good interpersonal skills and ability to train others
Experience with CATIA V5 is a strong plus
Experience with SolidWorks is a plus
Experience with SmarTeam is a plus
 
I found this other nugget about the desinging software on Business Wire:

Tesla Motors Selects Dassault Systèmes’ V6 PLM Solution. Tesla is beginning its upgrade to V6 with implementation of the ENOVIA V6 collaboration solution. Improvements and predefined templates in the latest ENOVIA release have enabled a successful pilot of the core solutions in just over a month, paving the way for a successful production implementation. Phase two of the implementation will involve the launch of the DELMIA digital manufacturing solution in late 2010. Future phases will migrate Tesla’s product authoring solution from CATIA V5 to CATIA V6. Additionally, Tesla is using the CATIA ICEM solution for Class A surfacing, DELMIA Human for occupant ergonomics, plus solutions from Dassault Systèmes software partners.

http://www.businesswire.com/news/home/20101027007368/en/Dassault-Syst%C3%A8mes-Reports-Strong-Quarter-Results-Licenses