Generate [min, max] range from strings

I need to generate a minimum and maximum value for a range of speeds (slow, medium, fast), the user can specify any combination of the 3 values, and receive a range that encompasses all.

Given any combination of "slow", "medium", "fast", getRange() will return a [min, max] range.

The function can be called in the following ways:

// Any combination of the values
getRange(['slow', 'medium', 'fast']); // [0, 100]
getRange(['medium', 'fast']); // [36, 100]
getRange(['slow']); // [0, 35]

// Or just a single string
getRange('fast'); // [76, 100]

// No string specified
getRange(); // [0, 100]

Is there any way to simplify my code? It seems a bit clunky and there's probably a simpler way of writing this:

function getRange(speed){
    var min, max;

    if(speed && speed.length){
        var obj = {};

        if(Array.isArray(speed)){
            speed.forEach(function(s){
                obj[s] = true;
            });
        }
        else{
            obj[speed] = true;
        }

        if(obj.slow){
            min = 0;
            max = 35;
        }

        if(obj.medium){
            if(typeof min == 'undefined'){
                min = 36;
            }
            max = 75;
        }

        if(obj.fast){
            if(typeof min == 'undefined'){
                min = 76;
            }
            max = 100;
        }
    }

    if(typeof min == 'undefined'){
        min = 0;
    }

    if(typeof max == 'undefined'){
        max = 100;
    }

    return [min, max];
};