My first thought was the aniti alias quality need s to go up a little as does the shadow quality.
i think you are using final gather too an it's a little blown out so try playing with the scale.
with HDRI you could most likely get away without any FG which would eleviate the shadow quality problem.
The displacement on the tyres could also do with a bit of work, you might even have to end up modelling the tread .
Hope this doesn't sound too critical as the model is good and could be great with some tweeking of the render settings.