This trophy can be started by navigating to Drive, selecting Tour, and choosing the event "Great Scott".
The third objective for this event is an average speed face-off, you have to beat the average speed of 88mph (Great Scott, Marty!) between two points.
It's quite late on in the lap, at least 3/4 of the way around, so it's a bit of a pain if you have to retry it a couple of times.
Of the cars you have available, the McLaren is probably your best bet, although it is possible with some of the other cars. You may have to level up quite a few times before this car is available so if you’re struggling with the other cars just come back to this when you’re a higher level and use the McLaren. Beat the 88mph average speed face off and the trophy will unlock.