Combine 2 methods into one with a discriminator variable

Car.cs

// Initial state for unifying two similar methods to use a discriminator variable.
//
// Includes 2 methods (Drive_WithFuelMonitoring and Drive_WithAverageSpeedCalculation)
// that are similar in intent and differ only in implementation. This may be a bug or
// may be intentional. We can't tell without asking the domain expert. So we want to
// unify them mechanistically.
//
// The history shows how.

using System.Collections.Generic;
using System.Linq;
using JetBrains.Annotations;

namespace Blog_gists
{
	public class Car
	{
		[NotNull] private readonly List<TravelSegment> drivingHistory = new List<TravelSegment>();

		public Car()
		{
			Odometer = 0;
			IsAtDestination = false;
		}

		public void Drive_WithFuelMonitoring(int speed, int distance)
		{
			for (var i = 0; i < distance; ++i)
			{
				DriveOneMile(speed);
			}
			FinishDriving();
			FuelRemaining -= CalculateFuelConsumed(speed, distance);
			Odometer += distance;
		}

		public void Drive_WithAverageSpeedCalculation(int speed, int distance)
		{
			for (var i = 0; i < distance; ++i)
			{
				DriveOneMile(speed);
			}
			FinishDriving();
			Odometer += distance;
			drivingHistory.Add(new TravelSegment(speed, distance));
		}

		public decimal? AverageSpeed
		{
			get
			{
				if (drivingHistory.Count == 0) return null;
				var totalDriveTime = drivingHistory.Select(h => h.Distance/h.Speed).Sum();
				return Odometer/totalDriveTime;
			}
		}

		public void FillGasTank(decimal numGallons)
		{
			FuelRemaining += numGallons;
		}

		private void FinishDriving()
		{
			IsAtDestination = true;
		}

		public class TravelSegment
		{
			public decimal Speed;
			public decimal Distance;

			public TravelSegment(int speed, int distance)
			{
				Speed = speed;
				Distance = distance;
			}
		}

		private decimal CalculateFuelConsumed(int speed, int distance)
		{
			return 3;
		}

		private void DriveOneMile(int speed)
		{
			IsAtDestination = false;
		}

		public int Odometer { get; private set; }

		public decimal FuelRemaining { get; private set; }

		public bool IsAtDestination { get; private set; }
	}
}

CarsDrive.cs

using FluentAssertions;
using NUnit.Framework;

namespace Blog_gists
{
	[TestFixture]
	public class CarsDrive
	{
		private const int Speed = 50;
		private const int Distance = 100;

		[Test]
		public void DrivingWhileUpdatingFuelGuageShouldMoveTheCarAndRequireGas()
		{
			var testSubject = new Car();
			testSubject.FillGasTank(12);
			testSubject.Drive_WithFuelMonitoring(Speed, Distance);
			testSubject.IsAtDestination.Should().BeTrue();
			testSubject.Odometer.Should().Be(100);
			testSubject.FuelRemaining.Should().Be(9);
			testSubject.AverageSpeed.Should().NotHaveValue();
		}

		[Test]
		public void DrivingWhileUpdatingMilageShouldMoveTheCarAndCalculateMilage()
		{
			var testSubject = new Car();
			testSubject.FuelRemaining.Should().Be(0);
			testSubject.Drive_WithAverageSpeedCalculation(Speed, Distance);
			testSubject.IsAtDestination.Should().BeTrue();
			testSubject.Odometer.Should().Be(100);
			testSubject.FuelRemaining.Should().Be(0);
			testSubject.AverageSpeed.Should().Be(50);
		}
	}
}