Nice installation!
You asked about mph display vs. the odometer.
For what it's worth, my ScanGauge also shows a bit less speed than my HX's speedometer shows. I think the speedo's are probably set to read a bit higher than actual.
For a nice tire size calculator, go here. Will show you the % correction and dimensional differences between various tire sizes. A very handy tool.
http://www.wickedbodies.net/Tire-Size-Calculator.htm