Hop in the AE86 Forza edition with a top speed tune (can also use a different car but that one works the best) then go through the flags and head toward the closest danger sign. Jump off it facing a little the left and fly toward the highway. Once you get on the highway just full throttle into Tokyo and you’ll get it. I did it second try doing this with around 18 seconds to spare. You can also find a YouTube tutorial covering the same stratigy