The way I read it, petroleum makes up 96% of all transportation inputs, and natural gas 2%. To completely replace petroleum with ng, you'd have to increase by 48 times the amount of ng provided to transportation. And since 3% of current production is going there, you'd have to have 144% of current ng production just for transportation.
Add back the 97% of current production going to industrial, residential/commercial and electric, and you need 241% of current ng production to replace petroleum. Even assuming you could produce that much, and distribute it, and cars could be modified to run on it ... what would that do to the depletion curve?