Make "Spacing variation in a,b" = zero and use the "Object spacing a,b" the way you need it
together with the object size in the population or in the object node itself Hannes.
I used this for the gears animation together with controlling the size of the population itself.
Edit: Just to be clear i was talking about only one car and its wheels.
Sometimes my second language handicap kicks too much in
But even with 3 cars or more you could still use one population with a density shader
for the wheels and another one for the middle parts if they are moving in the same direction.