Was able to modify your code to get it working. Thanks for the help.
private void textBox_End_Miles_TextChanged(object sender, EventArgs e)
{
int End_Miles_Value = Convert.ToInt32(textBox_End_Miles.Text);
int Start_Miles_Value;
if (Int32.TryParse(label_Start_Miles1.Text, System.Globalization.NumberStyles.Number, null, out Start_Miles_Value) && End_Miles_Value > Start_Miles_Value)
label_Miles_Drove1.Text = (End_Miles_Value - Start_Miles_Value).ToString();
}