I'm trying to oompute miles driven and miles per gallon when the user inputs starting mileage, ending mileage, and gallons of gas used. I need to do this in a loop so they can enter info for up to 3 tanks. I can't get the miles driven to compute. I know the miles driven will equal the ending mileage minus the previous starting mileage but I can't get it to hold the new values in the loop.

Here's what I have so far:

do
{
cout << "Enter the gallons used (Enter -1 to End): ";
cin >> gallonsUsed;
totalGallonsUsed = (totalGallonsUsed + gallonsUsed);

cout << "Enter the ending mileage: ";
cin >> endingMileage;

milesDriven = (endingMileage - milesDriven);
cout << "Miles driven: " << milesDriven << endl;

totalMilesDriven = totalMilesDriven + milesDriven;

milesPerGallon = (milesDriven / gallonsUsed);
cout << "The miles/gallon for this tank was: " << milesPerGallon;

overallMilesPerGallon = (totalMilesDriven / totalGallonsUsed);

Heard about arrays?
http://www.cprogramming.com/tutorial/lesson8.html