To find timer splits is rough, but the tracks that I find the best success with is the GT and the Top Gear Track.
The GT one has a very long front stretch with a nice starting line just before the first turn. I don't know the actual distance, but from that starting line to the actual starting line (you'd be going in the reverse direction, just to let you know) is a good distance and I can run that with some good 6-8 second times with varying cars.