Via the streaming API.
Try this:
Code:
curl --user USERNAME:TOKEN https://streaming.vn.teslamotors.com/stream/VEHICLE_ID/?values=speed,odometer,soc,elevation,est_heading,est_lat,est_lng,power,shift_state
where USERNAME is your MyTesla username, TOKEN is the first token returned in your vehicle's JSON returned from the REST API, VEHICLE_ID is the "vehicle_id" returned in vehicle's JSON. The TOKEN value rotates every few minutes, so you will need to query your vehicle's JSON description before opening a new stream.
Fields are returned as a comma separated list of values matching the field references in the URL query string. The first field is a timestamp.
Now run that curl command, then hop in your vehicle and drive around for a while, and watch it stream.
For those that use Ruby, I've already developed a ruby gem that supports both the rest and streaming APIs as well as a high-level Vehicle Object API. I'll release a public version by the end of the month, just need some time to clean it up a little (and I'm traveling quite a bit this month). Here's some sample code (from my twitter client) using the API:
Code:
portal = Teslamatic::API::Portal.new(config[:username], config[:password])
vehicles = Teslamatic::Vehicle.vehicles(portal)
vehicle = vehicles.first
current_gear = vehicle.gear_selector
current_odometer = vehicle.odometer