With the Tesla Model X set to launch in the US before the end of 2014, we knew it was just a matter of time before a test mule was spotted on public roads.

Spotted it was, by several curious onlookers, including one who snapped a photo (image via Reddit) of it near the University of California

What you see here is a Tesla Model S fitted with additional weight up top to simulate the raised center of gravity of the Model X.

Those curious looking objects attached to the wheels allow Tesla to fine tune the AWD system that comes as standard on the Model X.  We're certain there's additional instrumentation within to calculate body movement and other parameters that Tesla will have to focus on in developing the Model X.

These images confirm that development of the Model X is moving along.  Once the powertrain is finalized and the suspension is tweaked to cope with the added weight, the skateboard platform will be married to the Model X body and then tested again.

Ignore the "autonomous" reference in the title of the video, as that's clearly inaccurate:

More info, including additional images of the Model X test mule, can be found at this Tesla Motors Club thread.

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