Understanding differences between standard and High Prim/Mesh Physics
SL doesn't necessarily explain very well how these work so here is my attempt to explain based on my experience.
Original 32 prim physics
Originally vehicles built with regular prims and sculpties were limited to 32 prims. These prims are all phsyics enabled and all prims are solid surfaces that will not pass through roads, walls, etc. This size limitation is beneficial, you can get many vehicles on one sim for a contest or race without causing too much loading slowdown for everyone. Each prim counts as one land impact, including detailed sculpties.
The ACS scripts by default set the linkset 32 prims or under in standard physics mode. (assigning all links as standard prim shape) Build as normal if not using mesh prims.
Mesh/"High Prim" Physics.
This new physics style has its benefits. For traditional sculpty-based builders it allows a vehicle to have more prims or to incorporate mesh parts. This is slightly more complex to work with and takes a little extra setup for vehicles. For any vehicle with mesh in the linkset or more than 32 prims, this new physics mode is required and will apply to the entire linkset including sculpties and regular prims. The limitations are slightly obscure; you can have up to 255 individual prims in a vehicle linkset but your parts set to "solid"/convex hull cannot themselves exceed 32 land impact prims (the original vehicle limit). The root in a new physics linkset is always convex hull, so your root prim should be as simple as possible, as the complexity of the a convex hull prim dicatates its land impact score.
Any mesh in the linkset requires the new physics, even if less than 32 prims. SL has not yet provided a way to detect mesh in a linkset, so the ACS scripts currently are set to "switch over" to new physics when counting more than 32 individual prims. If you have mesh in the linkset and less than 32 individual prims, you will need to force on Mesh/High Prim mode at the top of the Settings script. You can also disable the ACS automatic physics settings there if you prefer to set up your prims manually.
Mesh prims (and othe prims in a mesh linkset) can bet set to have a convex hull shape or no shape. In convex hull mode, a prim is solid, and able to react to physics. No shape prims are like phantom prims, they have no weight and solid objects will pass through them.
Prims set to convex hull/"solid" use more land impact than no-shape ones depending on their complexity, and the more convex hull prims you have, the higher the land impact. The vehicle linkset limitation is still 32 prims for convex hull score.
If you have convex hull mesh tires that are 8 land impact each, you will not be able to use four in a linkset with them as solid/convex hull (counting the root you have exceeded 32 convex hull prim score) , but there is a way around this.
The root prim in the linkset is ALWAYS convex hull. So if you use an invisible cube for your root instead of a detailed mesh or sculpt, you will lower your land impact score and have more convex hull limit available to use elsewhere in the build.
You can make this cube fairly large to give the vehicle more physics weight to help make up for the fact your no-shape parts are without physics weight.
You need convex hull parts to hold the vehicle off the ground, usually just tires are needed to be set to solid/convex hull. If these are sculpted/mesh detailed tires then they will use up a lot (or all or too much) of your convex hull allotment. You may need to use invisible simple dummy prims to actually handle the convex hull part of the job of holding the vehicle up on a road, such as regular cylinder prims, and leaving the sculpted/mesh tires as no-shape. By adding simple invisible prims for convex hull/solid jobs, you can lower your total impact score.
Mesh/high prim vehicles also have the issue of their no-shape prims getting stuck on walls, edges etc, so an outer invisible shield prim is often the best way around this. Setting that to convex hull/solid instead of body parts and detailed frames, will also keep your land impact down.
No-shape prims do not have weight or physics resistance, so your 33 prim bike is going to be a lot lighter than your 32 prim standard physics bike where everything is solid. You may need to adjust things like turning strength and wheelie strength in new physics, because your vehicle is actually a lot "lighter" when comprised of many no-shape prims.