using System;

public class Program

{

    public static void Main()

    {

        string unit = "mi"; // km/mi

        double start = 120100, end = 120228, fuelUsed = 6; // 120100, 120306/120228, 27/6

        ConsumptionCalculator c = new ConsumptionCalculator(unit, start, end, fuelUsed);

        Console.WriteLine(c.ToString());

    }

}

​

/// <summary>

/// 

/// </summary>

/// <param name="a"></param>

/// <param name="b"></param>

/// <returns></returns>

public delegate double TwoInput(double a, double b);

​

/// <summary>

/// 

/// </summary>

public class ConsumptionCalculator

{

    TwoInput DistancePerUnit = (a, b) => a / b;

    static readonly double KM_MI_FACTOR = 1.60934;

    static readonly double MI_KM_FACTOR = 0.62;

    static readonly double LTR_GAL_FACTOR = 4.54609;

    static readonly int RND = 2;

    double consumption = 0;

    double altConsumption = 0;

    string distanceUnit, altDistanceUnit, volumeUnit, altVolumeUnit;

    double distanceTravelled, altDistanceTravelled, startOdoReading, endOdoReading, volumeConsumed, altVolumeConsumed;

    /// <summary>

    /// 

    /// </summary>

    public enum DistanceUnit

    {

        km,

        mi

    }

    /// <summary>

    /// 

    /// </summary>

    public enum VolumeUnit

    {

        ltr,

        gal

    }

    /// <summary>

    /// 

    /// </summary>

    /// <param name="distanceUnit"></param>

    /// <param name="startOdo"></param>

    /// <param name="endOdo"></param>

    /// <param name="volume"></param>

    public ConsumptionCalculator(string distanceUnit, double startOdo, double endOdo, double volume)

    {

        this.startOdoReading = startOdo;

        this.endOdoReading = endOdo;

        this.volumeConsumed = volume;

        distanceTravelled = endOdoReading - startOdoReading;

        switch (distanceUnit)

        {

            case "km":

                KmPerLtr();

                break;

            case "mi":

                MiPerGal();

                break;

            default:

                break;

        }

    }

    private void KmPerLtr()

    {

        distanceUnit = DistanceUnit.km.ToString();

        volumeUnit = VolumeUnit.ltr.ToString();

        altDistanceUnit = DistanceUnit.mi.ToString();

        altVolumeUnit = VolumeUnit.gal.ToString();

        altDistanceTravelled = Math.Round(distanceTravelled / KM_MI_FACTOR, RND);

        altVolumeConsumed = Math.Round(volumeConsumed / LTR_GAL_FACTOR, RND);

        Consumption();

    }

    private void MiPerGal()

    {

        distanceUnit = DistanceUnit.mi.ToString();

        volumeUnit = VolumeUnit.gal.ToString();

        altDistanceUnit = DistanceUnit.km.ToString();

        altVolumeUnit = VolumeUnit.ltr.ToString();

        altDistanceTravelled = Math.Round(distanceTravelled / MI_KM_FACTOR, RND);

        altVolumeConsumed = Math.Round(volumeConsumed * LTR_GAL_FACTOR, RND);

        Consumption();

    }

    private void Consumption()

    {

        consumption = Math.Round(DistancePerUnit(distanceTravelled, volumeConsumed), RND);

        altConsumption = Math.Round(DistancePerUnit(altDistanceTravelled, altVolumeConsumed), RND);

    }

    /// <summary>

    /// 

    /// </summary>

    /// <returns></returns>

    public override string ToString()

    {

        return string.Format("You've travelled {0} {1} ({2} {3}) and consumed {4} {5} ({6} {7}) in doing so, which is {8} {1}/{5} ({9} {3}/{7})", 

                                distanceTravelled, distanceUnit, altDistanceTravelled, altDistanceUnit, volumeConsumed, volumeUnit, altVolumeConsumed, altVolumeUnit, consumption, altConsumption);

    }

}