Budget category explanation: transportation

Under this category falls every conceivable way we spend money on our cars.  I include everything from insurance to gas to taxes to repairs.  Everything goes into our short- term savings account and is drawn out as necessary.  The gas money number is sort of odd because it is taken from the number of miles driven over the past year (I write down each car's odometer at the beginning of each month), divided by 26 (biweekly) divided by the cars average mileage (which I update occasionally), multiplied by the current cost per gallon of gas.  Whew!  It is a very accurate figure for our budget, though it does fluctuate according the the cost of gas.  Now, since this is taken from the mileage over the whole year, which includes out of town trips and vacations, we don't usually use the whole gas budget each month.  The extra is left in savings to be drawn from when we do go on vacation.  One figure that I would like to have more wiggle room on is the repair maintain and replace line.  While we probably have enough money in savings to repair and maintain, we certainly don't have enough to replace one of our cars if we had to.  If it came down to it, we would probably have to use emergency savings or tax return money or whatever we could find at the moment.  That is one area I could stand to improve on.