4.6 bar is quite high...
Tires also work as suspension. If you pump them that hard you'll increase wheel travel; the shock absorbers will generate extra resistance.
The rougher the road the lower the optimal pressure is. For a very light car and a standard tarmac road I think 3 bar is optimal, unless you run on space savers 
That said, if you prove me wrong I might pump mine beyond 3 bar 
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2011 Honda Insight + HID, LEDs, tiny PV panel, extra brake pad return springs, neutral wheel alignment, 44/42 PSI (air), PHEV light (inop), tightened wheel nut.
  lifetime FE over 0.2 Gigameter or 0.13 Megamile.
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