A good "barrier" is 4k rpms. I NEVER rev past 4k rpms on a normal commute. Simply the car makes enough power between 3k and 4k to give you enough "go" to stay with the flow of traffic. If you're trying to get a lot of miles out of your gas shift @ 3k rpms but it'll be a pain if you're trying to go anywhere quick. Plus you'll probably get angry drivers honking horns because you're going so slow. Here's a list of speed limit and my gear.
20 mph = 2nd/3rd gear (depends on how far I have to go, etc).
25 mph = 3rd gear
30 mph = 4th gear
35 mph = 4th gear
40 mph = 5th/6th gear
45+ mph = 6th gear
Pretty much when you're cruising keep it under 2500 rpms. When you're getting up to speed don't shift past 4k rpms. That's how I drive anyways. I like to give the car a good amount of gas in 1st gear to 4k rpms then shift to 2nd... car sounds amazing (berk cbe). I seriously need to stop driving with my windows down my gas mileage gets crappy.
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Good drivers just drive. 
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