YAY! (lol) Ya so u have to break out of seattle, and then turn to the left, through the new stadium, and then turn left again, then drive down that back straight close to the wall without being sucked back into the track. You should hit a spot where your car will pop into the air and your speeds will increase rapidly.
(Did that make any sense at all?)
PS-I did find weird things other than the speed glitch. I once popped like 500 feet into the air and fell all the way back down to seattle. and another time, the nose of my car kinda sunk into the ground and my car was about at a 60 or 70 degree angle. Everytime I would accelerate, the car would barrel roll twice. Weird things happen outside of seattle!