tankdrive code from http://www.goodrobot.com/en/2009/09/tank-drive-via-joystick-control/

tankdrive.js

// Tankdrive function, that converts the x and y values
// to store as left/right
function tankdrive(x, y) {
    // first Compute the angle in deg

    // First hypotenuse
    var z = Math.sqrt(x*x + y*y);
    // angle in radians
    rad = Math.acos(Math.abs(x)/z);
    // and in degrees
    angle = rad*180/Math.PI;

    // Now angle indicates the measure of turn
    // Along a straight line, with an angle o, the turn co-efficient is same
    // this applies for angles between 0-90, with angle 0 the co-eff is -1
    // with angle 45, the co-efficient is 0 and with angle 90, it is 1
    var tcoeff = -1 + (angle/90)*2;
    var turn = tcoeff * Math.abs(Math.abs(y) - Math.abs(x));
    turn = Math.round(turn*100)/100;

    // And max of y or x is the movement
    var move = Math.max(Math.abs(y),Math.abs(x));

    // First and third quadrant
    if( (x >= 0 && y >= 0) || (x < 0 &&  y < 0) ) {
        left = move;
        right = turn;
    } else {
        right = move;
        left = turn;
    }

    // Reverse polarity
    if(y < 0) {
        left = 0 - left;
        right = 0 - right;
    }
}

tankdrive.rb

# Tankdrive code converted to Ruby

def motor_speeds(x, y)
	return 0,0 if x == 0 && y == 0
	# first Compute the angle in deg

	# First hypotenuse
	z = Math.sqrt(x*x + y*y)

	# angle in radians
	rad = Math.acos(x.abs / z)

	# and in degrees
	angle = rad * 180 / Math::PI

	# Now angle indicates the measure of turn
	# Along a straight line, with an angle o, the turn co-efficient is same
	# this applies for angles between 0-90, with angle 0 the co-eff is -1
	# with angle 45, the co-efficient is 0 and with angle 90, it is 1
	tcoeff = -1 + (angle / 90) * 2
	turn = tcoeff * (y.abs - x.abs).abs
	turn = (turn * 100).round / 100

	# And max of y or x is the movement
	move = y.abs > x.abs ? y.abs : x.abs
	# First and third quadrant
	if ( (x >= 0 && y >= 0) || (x < 0 &&  y < 0) )
			left = move
			right = turn
	else
			right = move
			left = turn
	end

	# Reverse polarity
	if (y < 0)
		left = 0 - left
		right = 0 - right
	end

	return left, right
end