Production speed effect on quality

[Frankschtaldt]

I'm wondering if anyone has a good idea (or if Eric is willing to explain) if the "Production speed" slider in the factory window has any effect on perceived quality of the vehicles you produce or if it only effects the number of cars you can make per line?
I've always turned it towards quality rather than quantity when making high end vehicles but I've seen no real evidence that I'm actually gaining anything from doing that.