Okay, so this, to me, sounds like what the OP is doing--reminding him of his driving flaws when she is with him--somehow she is a nag/harpy. And you're lucky that your DH listens to your request. So, what if he didn't? I think "nags" are created not born!
Also, as I stated, I'm married to the tailgater. I've told him the same thing--"don't drive like that while the family is in the car; do whatever you want to yourself" but, you know, he seems to not be able to control himself. So do I keep my mouth shut, refuse to get in the car with him, remind him again? I feel like whatever I might do about *his* bad behavior somehow makes me look like a shrew. Oh, and my husbands "tailgating" has caused three tickets and two car accidents. Do you think I have the best insurance rate?