Fuzzy Math?

In everything I read lately, there's an article for tips on how to save on gas. One that I read stated the following:

Take more time! Instead of driving at 70 mph, drive at 60 mph and you'll use less gas!

I don't get it.

Really, someone please explain this to me.

If I drive at 70 mph, I'll get there quicker.
If I drive at 60 mph, I'll get there later, but I'll be using gas for 10 more minuets per 60 miles.

Won't I use the same amount of gas no matter how fast or slow I go? I mean, if I'm going to save gas driving at 60 mph, then why wouldn't I save that much more driving at 40 mph?

Doesn't quite add up to me. Besides, there just isn't enough time in the day to be driving slower.