By using the PCJ-600 from "9mm Mayhem" mission. It is unique in a way that it doesn't sink in the water. Instead, it just falls through it into the underworld. Using it, and knowing where the Subway tunnels are, I managed to use the roof of the Subway tunnel to get to Staunton. Getting to Portland was easier, as just riding off at full speed towards Portland from that pier where the Speeder is parked will spawn you in Chinatown.
I know that the Speeder is spawn right above the Subway tunnel. But I still don't know where exactly there should I look for this loophole.
Another question I have: How does the Car Salesman mission have to be completed? All 6 levels in one go? Or I can abort the mission whenever I like? Or I can abort the mission only at certain points in order not to lose progress?