As the guesswork surrounding driving speeds in S&T seems to be a widespread annoyance, I laid out some routes in S&T on different road types and set the slider to different values to see how the travel time changed. I then did the math to deduce what speeds S&T was using for each slider setting for each road type. There's a minor amount of guesswork involved because S&T only shows whole minutes on travel time--- no seconds. But on the whole, this should be pretty close:
Limited access (eg non-urban Interstate):
Average speed = 69mph
+1 slider unit = 75
+2 = 78
+3 = 82
Major Road (eg major numbered state highway in open country, eg Rt 16 in Ossippee NH)
Average speed = 50mph
+1 slider unit = 53
+2 = 55
+3 = 58
"Other Highway" (eg Route 1a in the suburban north shore of Mass)
Average speed = 35mph
+1 slider unit = 36
+2 = 37
+3 = 38
Arterial Roads (eg Route 9 into Boston)
Average speed = 50mph
+1 slider unit = 51
+2 = 52
+3 = 53
Other Streets (eg suburban town road)
Average speed = 22mph
+1 slider unit = 23
+2 = 24
+3 = 25
Limited access (eg non-urban Interstate):
Average speed = 69mph
+1 slider unit = 75
+2 = 78
+3 = 82
Major Road (eg major numbered state highway in open country, eg Rt 16 in Ossippee NH)
Average speed = 50mph
+1 slider unit = 53
+2 = 55
+3 = 58
"Other Highway" (eg Route 1a in the suburban north shore of Mass)
Average speed = 35mph
+1 slider unit = 36
+2 = 37
+3 = 38
Arterial Roads (eg Route 9 into Boston)
Average speed = 50mph
+1 slider unit = 51
+2 = 52
+3 = 53
Other Streets (eg suburban town road)
Average speed = 22mph
+1 slider unit = 23
+2 = 24
+3 = 25