What speed is the slowest I can legally and safely drive on the freeway?

I hate spending money at the pump, and while I do have a pretty efficient vehicle ('01 Honda Civic 5-Spd SOHC VTEC), I'd like to keep gas spending to a minimum. I typically drive 59 MPH (when my tachometer reads 2800 RPM) on the furthest right lane. I went over to FuelEconomy.gov and used their little calculator and discovered I could potentially save upwards of $0.52/gallon if I reduce my speed 10MPH. So how slow is illegal and potentially dangerous? My (informal) understanding is the highway speed should be /- 10MPH of speed limit.

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