The wheels spawn where the admin sets them to spawn in types. The default types for wheels looks like this. So that would be on the ground in industrial areas.

<type name="vg7_wheel">
<nominal>2</nominal>
<lifetime>2700</lifetime>
<restock>0</restock>
<min>1</min>
<quantmin>-1</quantmin>
<quantmax>-1</quantmax>
<cost>100</cost>
<flags count_in_cargo="0" count_in_hoarder="0" count_in_map="1" count_in_player="0" crafted="0" deloot="0"/>
<category name="vehiclesparts"/>
<tag name="floor"/>
<usage name="Industrial"/>
</type>