I've cut dozens of perfect trees with the Vipers and ZR-1, even beaten faster cars.
The RT timer STARTS when the 3rd amber light comes on, with a heavy vehicle (X5) or MT trans vehicle, this is the time to GO!!!!
As stated prior, DON'T ROLL OUT UNTIL THE PRE-STAGE LIGHT GOES OUT (deep stage of vehicle). Shallow staging allows for a "running start/weight transfer" prior to breaking the beam that starts the timer!!! This allows for the LOWEST possible ET the vehicle can run.
More details........
The Staging Light -- Drag Racing