How to program thresholds?
Thanks again for your contributions.

I've gone ahead and decided to go with the following code. Its purpose is to turn vol up or down based on current car MPH. I choose to define brackets that cover a range of values. When one bracket ends, another begins. I've also set the previous bracket to current bracket at the end of each loop so that the application knows where it was previously, so that if it hops above or goes under a defined bracket, it does the desired action.

I've used a random generator to supply the MPH for testing purposes for now and commented out a few things that disturbed the testing process. If you'd like to critique this code to make is better, please let me know. 

Note: I've also borrowed some of this code from another individual (Matthew McMillan, http://matthewcmcmillan.blogspot.com) who wanted to figure out how to convert a vehicle speed sensor wire info into MPH. I needed this code in order to make use of the vehicle's speed.

#include &lt;SPI.h&gt;
#include &lt;Wire.h&gt;

const int lightPin = 0;
const int hardwareCounterPin = 5;
const int samplePeriod = 1000; //in milliseconds
const float pulsesPerMile = 4000; // this is pulses per mile for Toyota. Other cars are different.
const float convertMph = pulsesPerMile/3600;
unsigned int count;
float mph;
unsigned int imph;
int roundedMph;
int previousMph;
int prevCount;

// Declare brackets
const int BracketA = 1;
const int BracketB = 2;
const int BracketC = 3;
const int BracketD = 4;
const int BracketE = 5;
const int BracketF = 6;

// Initialize Previous and Current Brackets
int PreviousBracket = 1;
int CurrentBracket = 1;


void setup(void) {
  Serial.begin(9600);

  TCCR1A = 0; //Configure hardware counter 
  TCNT1 = 0;  // Reset hardware counter to zero
}

void loop() {


  /////////////////////////////////////////////////////////////
  // This uses the hardware pulse counter on the Arduino.
  // Currently it collects samples for one second.
  //
  bitSet(TCCR1B, CS12); // start counting pulses
  bitSet(TCCR1B, CS11); // Clock on rising edge
  delay(samplePeriod); // Allow pulse counter to collect for samplePeriod
  TCCR1B = 0; // stop counting
  count = TCNT1; // Store the hardware counter in a variable
  TCNT1 = 0;     // Reset hardware counter to zero
  mph = (count/convertMph)*10; // Convert pulse count into mph.
  imph = (unsigned int) mph; // Cast to integer. 10x allows retaining 10th of mph resolution.

  int x = imph / 10;
  int y = imph % 10;

  // Round to whole mile per hour
  //if(y &gt;= 5){
  //  roundedMph = x + 1;
  //}else{
  //  roundedMph = x;
 // }


  //Code to write your own MPH for testing purposes.
  //if (Serial.available() &gt; 0) {
                // read the incoming byte:
               // roundedMph = Serial.read();
               //}

  //If mph is less than 1mph just show 0mph.
  //Readings of 0.9mph or lower are some what erratic and can
  //occasionally be triggered by electrical noise.
  //if(x == 0){
  //  roundedMph = 0;
 // }

  roundedMph = random(1,80);


  // Don't display mph readings that are more than 50 mph higher than the 
  // previous reading because it is probably a spurious reading.
  // Accelerating 50mph in one second is rocketship fast so it is probably
  // not real.
  if((roundedMph - previousMph) &gt; 50){
    roundedMph = previousMph;
  }


  //Place the gatherered MPH value into a bracket
  //---------------------------------------------
  if(roundedMph &gt;= 0 &amp;&amp; roundedMph &lt; 20) {
   CurrentBracket = BracketA;  //Value1
   Serial.println("I Choose Bracket A");
    }

  if(roundedMph &gt;= 20 &amp;&amp; roundedMph &lt; 30) {
   CurrentBracket = BracketB;  //Value2
   Serial.println("I Choose Bracket B");
    }

  if(roundedMph &gt;= 30 &amp;&amp; roundedMph &lt; 40) {
   CurrentBracket = BracketC;  //Value3
   Serial.println("I Choose Bracket C");
    }

  if(roundedMph &gt;= 40 &amp;&amp; roundedMph &lt; 50) {
   CurrentBracket = BracketD;  //Value4
   Serial.println("I Choose Bracket D");
    }

  if(roundedMph &gt;= 50 &amp;&amp; roundedMph &lt; 65) {
   CurrentBracket = BracketE;  //Value5
   Serial.println("I Choose Bracket E");
    }

  if(roundedMph &gt;= 65) {
   CurrentBracket = BracketF;  //Value6
   Serial.println("I Choose Bracket F");
    }


 //Decide if bracket stayed the same, went up, or down. 
 //--------------------------------------------------- 
 if(CurrentBracket == PreviousBracket) {
 //Do Nothing
 Serial.println("Nothing Changed");
 }  

if(CurrentBracket &gt; PreviousBracket) {
 //Command for Vol Up
 int VolUp;
 VolUp = CurrentBracket - PreviousBracket;
Serial.println("Volume Went Up One by "); 
Serial.println(VolUp);
delay(2000); // Delay to not change Vol to fast if near threshold 
}

if(CurrentBracket &lt; PreviousBracket) {
  //Command for Vol Down
  int VolDown;
  VolDown = PreviousBracket - CurrentBracket;
 Serial.println("Volume Went Down One by ");  
 Serial.println(VolDown);
 delay(2000); // Delay to not change Vol to fast if near threshold
 }
  previousMph = roundedMph; // Set previousMph for use in next loop.
  PreviousBracket = CurrentBracket; //Set PreviousBracket for use in next loop.
}