My philosophy on this is that driving 80 instead of 70 saves me 10 miles for each hour I am on the road. That translates to 8.5 minutes. Is 8.5 minutes worth the increased gas consumption and the increased stress of driving 80 instead of the speed limit? I can envision a few cases where it might be, but generally I poke along at the speed limit. I find it more restful, more economical, and the time saved is not worth the extra hassle. Your mileage may vary (pun intended). If traffic is moving at 80 and I feel like I'm impeding traffic flow, I might drive a bit faster. Or not.