Better vehicle dragging (#72515)

* Better pushing, with granularity

* better variable names

src/grab.cpp

@@ -74,14 +74,18 @@ bool game::grabbed_veh_move( const tripoint &dp )
 
     //vehicle movement: strength check. very strong humans can move about 2,000 kg in a wheelbarrow.
     int mc = 0;
-    int str_req = grabbed_vehicle->total_mass() / 100_kilogram; //strength required to move vehicle.
+    // worst case scenario strength required to move vehicle.
+    const int max_str_req = grabbed_vehicle->total_mass() / 10_kilogram;
+    // actual strength required to move vehicle.
+    int str_req = 0;
     // ARM_STR governs dragging heavy things
     int str = u.get_arm_str();
 
     //if vehicle is rollable we modify str_req based on a function of movecost per wheel.
 
     const auto &wheel_indices = grabbed_vehicle->wheelcache;
     if( grabbed_vehicle->valid_wheel_config() ) {
+        str_req = max_str_req / 10;
         //determine movecost for terrain touching wheels
         const tripoint_bub_ms vehpos = grabbed_vehicle->pos_bub();
         for( int p : wheel_indices ) {
@@ -98,8 +102,10 @@ bool game::grabbed_veh_move( const tripoint &dp )
         }
         //finally, adjust by the off-road coefficient (always 1.0 on a road, as low as 0.1 off road.)
         str_req /= grabbed_vehicle->k_traction( get_map().vehicle_wheel_traction( *grabbed_vehicle ) );
+        // If it would be easier not to use the wheels, don't use the wheels.
+        str_req = std::min( str_req, max_str_req );
     } else {
-        str_req *= 10;
+        str_req = max_str_req;
         //if vehicle has no wheels str_req make a noise. since it has no wheels assume it has the worst off roading possible (0.1)
         if( str_req <= str ) {
             sounds::sound( grabbed_vehicle->global_pos3(), str_req * 2, sounds::sound_t::movement,