I have a manual switch in the dash to run both fans simultaneously. Some say running dual fans impedes airflow above 35mph. Is there any truth to this?

 

Whether it does or does not impede airflow, and manual switch or not, if everything is operating properly, then there shouldn't be any need to run the fans at all when you're moving at a steady cruising speed, let alone run them all the time.

Most likely, the things that will be hurt by running your fans all the time are the electrical system, the fan relays and fan motors. Then one day you might need those fans, but they won't be there to do their job because they'll have burned-out. And when(not if) that happens, don't curse your car, lol.

 

Along with a manual switch, I recommend wiring up a 3 relay system with temperature control switches to have the fans on at 1/2 speed at lower high temps and at full speed at higher high temperatures. Think of it as insurance or preventative maintenance.

https://www.thirdgen.org/forums/cool...an-wiring.html

 

Matthew, I have noticed on my `88 Iroc 383 TPI that it does run cooler at highway speeds with the fans off. In addition to the fan switch, both of my fans are on toggle switches so I can turn them on at any time and I also have an LED connected to each relay so I know when they are on.