Ive driven eco boxes for years and they are all like this. The 3 cylinder engine in the geo metro doubles as an air plane engine and those like many stationary engine run at 3600 rpms. So in effect that isnt too fast. Second like mentioned its in the power band. This car is one of very few I think nailed the gearing. It can haul ass from a dead stop and I love the extra power in top gear vs having the additional step of down shifting above highway speeds to go around a slower driver merge onto another section of road, etc.
Ive driven a few eco boxes with 6th and the rpm change was maybe 500 tops it seemed like the additional gear was wasted vs having an extra high over drive one. On some trucks they have 2 over drives one for towing and one for no load.
In my S the display reads just over 40 for mpg and I frequent 70+ mph speeds.