Public Sub OverallAverage()
 
 
        Dim miles, gallons As Double
        Dim totalGallons As Double = 0
        Dim totalMiles As Double = 0
        Dim tank As Double = 0
        Dim average As Double = 0
 
 
        Console.Write("Enter the miles driven, use -1 to end: ")
        gallons = Console.ReadLine()
 
        While miles <> -1
 
            Console.Write("Enter the gallons: ")
            gallons = Console.ReadLine()
 
            tank = miles / gallons
            Console.WriteLine("The miles / gallon for this tank was " & tank)
 
            Console.WriteLine("Enter the miles driven, use -1 to end: ")
            miles = Console.ReadLine()
 
            totalGallons = totalGallons + gallons
            totalMiles = totalMiles + miles
 
        End While
 
        average = totalMiles / totalGallons
        Console.Write("The overall average miles/gallon was: " & average)
 
    End Sub

        'you had this as gallons should be miles
        miles = Console.ReadLine()

            'move this to after calculate totals otherwise changing miles 
before adding to total
            'miles = Console.ReadLine()

            totalGallons = totalGallons + gallons
            totalMiles = totalMiles + miles
            miles = Console.ReadLine()

miles=console.Readline()

If miles >= 0 Then
 
            average = totalMiles / totalGallons
            Console.WriteLine("The overall average miles/gallon was: " & average)
        Else
            Console.WriteLine("You have chose to quite by pressing -1")
 
        End If