Generally speaking, welding smoke will stratify at 12-15' but numerous factors can affect this. One fan in the wall or even ceiling for general exhaust is a slow way to remove the fumes, and you do have to let some air back in to the shop to exhaust some air out.
Your best solution is to capture the smoke or vehicle exhaust at their source.
Their are many good articulating fume arms out there, names like Nederman, Coral, I have an old worn out Torit (it was free!) mounted over my welding bench, and a long piece of 6" garage hose to connect between the hood on the arm and a vehicle's exhaust pipe.
Need a good blower, capable of 4" static pressure or more, not a furnace fan.
The idea is low volume (600 to 800 cfm) medium pressure.
This may cost more up front, but is a far more effective system than a wall fan at one end of the shop. I have one of those too, and rarely use it.
