After my last post, a few people pointed out that using heightmaps might be a better approach for this. So I scrapped the previous octree approach, and took a tangent to try out JangaFX's GeoGen, which offer ways to create realistic landscapes, and export meshes, heightmaps and textures out of them. Then that info is imported into the game and wheel collision code looks up the height (z) based on the (x,y) of the position, the rest of the code remains the same. Here's a demonstration of driving the car around the hills: