I recently took a trip to Memphis and looked up my route using Google maps (I was going to a campsite I'd never been to before). It told me it was 320 miles and would take 4.7 hours. Doing a little math, I came up with just over 68 mph. At 65 mph as I usually drive (better gas mileage) it would take 4.9 hours. Mind you, that is non-stop (no gas stops, no bathroom breaks, no stopping at the golden arches). So I planned on 5 1/2 hours. It took me about 10 minutes over that due to traffic and lower speed limits on some of the roads.
So if you use on-line maps, always allow extra time beyond what the information tells you. My general rule is to allow at least 30 minutes more for every 6 hours. That allows for slow traffic, gas stops, etc.
ALSO: before you go, verify the given directions with a map or atlas. Most on-line maps use primary roads for their routing and you can occasionally save time by selecting a secondary route. When I travel to Hattiesburg, Mississippi, I'm often routed along I-40 to Memphis, then down I-55 to Jackson, the along US 49. But if I go down I-530/US 65 from Pine Bluff, I save around 80 miles and almost an hour driving time. It's also much more scenic ;)
Bottom line: if you use on-line maps, always allow more driving time than the site gives you and don't be afraid to check for alternate routings. Most importantly , however, is keep a map or an atlas with you on your trip. Google won't tell you if a bridge if closed for repairs or if the road washed out. And occasionally, they get the route numbers wrong.