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In terms of the throttle control, a mechanical throttle generally consists of a cable that runs from the accelerator to the throttle body, and pulls on a cam lever that opens and closes the throttle body.
With an electronic throttle, the accelerator is attached to a potentiometer that sends an electronic signal to the ECU than then sends on a signal to a stepper motor on the throttle body to open it a given amount. The major advantage of electronic throttles is it allows for cruise control with little to no additional hardware. Unlike with mechanical throttles, which usually require a secondary cable, and a vacuum canister for auxiliary throttle control. As well, throttle response can be more fine-tuned, with the ECU able to make minute adjustments on-the-fly.
In response to your second question, a "high" GVWR is usually over 8,600. For instance, the Hummer H2, and 3/4 and 1-ton trucks are not required to submit fuel economy figures to the EPA. If the GVWR is over 8,600, it's considered a "heavy duty" vehicle. This information can usually be found on a plaque inside the driver's doorsill or on the door itself. Hope this helps.
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