In the Corebook I found a chart where it adds negative dice to piloting skill rolls made in difficult terrain for ground vehicles. It also mentions that terrain can slow speed, but it doesn't show any amount. Is this to be found elsewhere, or it is left to DJ criteria?

Difficult terrain increases the maneuver cost to change bands jjust as it does in personal scale. The All-Terrain Driver talent alllows an exception.

In the Corebook I found a chart where it adds negative dice to piloting skill rolls made in difficult terrain for ground vehicles. It also mentions that terrain can slow speed, but it doesn't show any amount. Is this to be found elsewhere, or it is left to DJ criteria?

Since the difficulty of a Piloting check is typically Speed upgraded by half Silhouette (plus setback dice for terrain) you might want to lower the speed voluntarily if the terrain gets too tricky. I suspect that's what that piece of text means. Or, possibly, that the GM can use Threat to force the Speed down.

Difficult terrain increases the maneuver cost to change bands jjust as it does in personal scale. The All-Terrain Driver talent alllows an exception.

I never thought about difficult terrain doubling the manoeuvre cost for vehicles before. That's very useful, not to mention that it makes All-Terrain Driver an awesome talent.