I commute from WI Dells to Madison five days a week, a 52 mile drive including 40 miles of interstate. Driving 40 miles at 65 miles per hour takes 36.9 minutes. The same drive at 75 miles per hour takes 31.9 minutes. I was going to make a point but apparently you're actually right. Fuck.
I usually set cruise at 74 (I've passed cops at 79 regularly) and I'm just fine, but occasionally I'll hit 80 if I'm trying to pass a really annoying semi (srsly, they're terrible) or 90 to avoid an accident or get the hell away from a tailgater. The 80, I could probably not get away with but I think I could justify the other stuff.
Going 5 over works well for 400 miles ... but you have to go long distances for speeding to make a significant difference.
For city driving ... you're better off exploring back roads and neighborhood roads as these don't typically have traffic lights. But
ALWAYS do a complete stop at a stop sign! It takes a little getting used to stopping for 1-2 seconds, but it pays off and becomes a good habit. Driving the speed limit also becomes a good habit when you get used to it. It may be hard for you to get used to it though.
Driving the speed limit is also a way to get tailgaters off your back as they typically speed. They seem to not fully grasp reality ... you don't need to take a physics course to feel the effects of momentum and inertia. What's worse is that they continue this behavior in the rain! Another possibility is that they actually believe the closer they are to the bumper, the faster they get to their destination ...
Anyway, I hate driving! I hate the way other people drive! Especially those dumb fucks who drink coffee and text while the car is in motion.
P.S. Always use two hands on the wheel. A bump or pothole can easily disorient a one-handed driver ... and in the right situation, that's a crash.