I built a tri-oval back awhile ago...
Mine was 16' long, and I recall 9' at the widest spot.

The end turns had normal lane spacing (3 1/2"), while in the D section, I squeezed the inner lanes towards the outside lanes. This not only made the inside lanes a bit longer, but also gave them a tighter radius which meant they had to back off a bit more than the outside lane. This even up the racing, and the outside lane could keep up with those on the inside. Also made for some great action in the turn, as there was a LOT of rubbing happening :auto-driving:

