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They didn't make it taller because in 5th you should already run out of power before you run out of rpm. No point in going further. Part of the problem may be that the vibrations make people feel like the bike is working "too hard" at cruise when it's really not. You should be over 5 at cruise. Best gas mileage would be obtained when you gear the bike so your cruising speed matches the rpm where peak torque happens. The xs doesn't hit peak torque until 6k.
redline is 7,500 so you're fine at 5. In the bottom left the lowest line is a chart of fuel consumption yamaha put in the xs1b manual. They've got the torque peak extending to 7k here, possible factors being that the 256 engines had both a hotter cam and the 38mm carbs, but most xs engines will peak at 6k
edit: I should probably be clear that this is a plot of the engine on a dyno and doesn't take into account speed of the bike/wind resistance etc. Speeding up from 5k to 6k on the road probably won't help your mileage but if you're re-gearing the bike setting it up to turn 6k at 70mph should be better for mileage than 3k at 70.