In today's high fuel price society, there is a rush to replace the old gas guzzler with a more efficient fuel usage vehicle; but, is such a replacement cost effective or might one do better by changing transportation habits until a new vehicle is absolutely needed.  One way to find out is to compare the current vehicle usage with the future vehicle usage and see if replacing will actually save money and how much.  The determination is simple although it does require some arithmetic to accomplish. 

First determine current usage.  This is done by reading the milage on the odometer (the actual name of the device that displays the milage), dividing it by the number of months owned and then multiplying by 12 to get the average yearly usage. 

Next is getting the miles per gallon.  You can use the rating of the vehicle which would give a general idea or obtain a number in line with the actual usage.  To get the latter, fill up the gas tank and note the odometer milage.  Note the odometer milage at the next fill up and make note of the gallons put in.  Subtract the first milage reading from the second milage reading, then divide by the gallons put in.  This will give you the miles per gallon for the period between fill ups (this only works when the tank is filled each time and may change between fill ups due to a change in driving needs during the different period). 

Now divide the yearly miles by the miles per gallon and the result is the gallons of gas used for the year on average.  Now take the same miles per year and divide it by the miles per gallon of the future vehicle.  The latter result would be a good estimate of the gallons used by the new vehicle.  Subtracting the new vehicle gallons from the old vehicle gallons will result in the gallons saved as related to actual vehicle usage based on past usage.  Multiply the difference by the cost of a gallon of gas and that is the actual savings based on driving habits to date. 

Dividing the final number into the net spent (cost of new vehicle less the trade in value of the old) and the result is the time span to the break even point of buying a new vehicle.  If the savings is not cost effective, it may be better to work on changing transportation habits than purchasing a new vehicle.