I don't think there is a 'right' answer - to some extent, it depends on your personal preferences.
If you want maximum performance available, even at times when you're not using it, then let the engine rev up to 4000 or more. If you're just cruising along a flat road in low speed traffic, then changing up below 2000 will be as much as you need.
Do not be afraid of revving the engine - many people do not rev them enough - but using more revs than you need does increase fuel consumption. 'Slogging' an engine, trying to make it deliver power at too low revs, isn't good for them, so change down if in doubt.
Do US cars not have the gearchange indicator? That is quite a sophisticated piece of work and it will tell you to change up when it thinks that the same performance or acceleration is available in the next higher gear. Equally it will tell you when you should change down because the speed is too low for the current gear - if you like, an 'anti-slogging' warning.
But most of all, do what feels fun.