I run west coast mirrors on my van sometimes I fold them in for banzai runs. I know definitely on back to back (with cool off between) runs, same air conditions, I went from an 11.80 to an 11.75 and picked up over 1.5 mph. The MPH gain might actually be more but can’t remember exactly.....but I have receipts showing gains from all the times I’ve folded them in. It helps absolutely