Hi, I'm trying to solve a problem for a car millage. What are my mistakes. Since I can't get the correct answer.

Enter the miles used (-1 to quit): 287
Enter gallons: 13
MPG this tankful: 22.076923
Total MPG: 22.076923
Enter the miles used (-1 to quit): 200
Enter gallons: 10
MPG this tankful: 20.000000
Total MPG: 21.173913
Enter the miles used (-1 to quit): 120
Enter gallons: 5
MPG this tankful: 24.000000
Total MPG: 21.678571
Enter miles (-1 to quit): -1

#include <iostream>
#include "conio.h"
using namespace std;
void main()

{
	cout.setf(ios::fixed);
	cout.setf(ios::showpoint);
	cout.precision(6);

	float miles = 0; float gallons = 0; float milage = 0; float TMG = 0; float TM = 0; float TG = 0;

	cout << "Enter Miles(-1 to quit): ";
	cin >> miles;
	while (miles != -1)
	{
		
		cout << "Enter gallons: ";
		cin >> gallons;
		milage = miles / gallons;
		cout << "Milage this tankful is: " << milage << endl;
		
		if (TMG == 0)
		{
			cout << "Total Milage is: " << milage << endl;
		}
		else
		{
			cout << "Total Milage is: " << TMG << endl;
		}
		
		TM = TM + miles;
		TG = TG + gallons;
		TMG = TM / TG;
		cout << "Enter Miles(-1 to quit): ";
		cin >> miles;
	}

	_getche();
}