'Declaration Station
        Dim CostPerAnnum As Decimal
        Dim MPG As Decimal 'Miles Per Gallon
        Dim MPA As Decimal 'Miles Per Annum
        Dim POP As Decimal 'Price of Fuel
        Dim POD As Decimal 'Price of Diesel
        Dim MPL As Decimal 'Miles Per Litre
        Dim LPA As Decimal 'Litres Per Annum
        Dim Car(10) As Decimal
        Dim MenuChoice As Integer
        Dim i As Integer = 0

Menu: Do While i < 11
            Console.WriteLine("Press 1 to enter a vehicles details")
            Console.WriteLine("Press 2 to see which is cheaper to run")
            MenuChoice = Console.ReadLine
*case statement here*
VehicleAdding: Car(i) = i + 1
            Console.WriteLine("Press 1 to add petrol details")
            Console.WriteLine("Press 1 to add diesel details")
*case statement here*
PetrolCars: MPG = Console.ReadLine
            MPL = MPG * 0.219969248 'Converting miles per gallon into miles per litre
            LPA = MPA / MPL 'Amount of litres used per year = MilesPerLitre/MilesPerAnnum
            Car(i) = POP * LPA 'PricePerAnnum (in pence) = PriceOfFuel x LitresUsedPerYear
            Car(i) = Car(i) / 100 'PricePerAnnum converted into pounds
Diesel: MPL = MPG * 0.219969248 'Converting miles per gallon into miles per litre
            LPA = MPA / MPL 'Amount of litres used per year = MilesPerLitre/MilesPerAnnum
            Car(i) = POD * LPA 'PricePerAnnum (in pence) = PriceOfFuel x LitresUsedPerYear
            Car(i) = Car(i) / 100 'PricePerAnnum converted into pounds

The compare bit:
        Dim Min As Decimal
        Dim i_min = LBound(Car)
        Dim i_max = UBound(Car)
        For i = i_min To i_max
            If Car(i) < Min Then Min = Car(i)
        Next