Set your car's trip odometer to zero the next time you fill your gas tank (consult your owner's manual for how to do this). Make a notation as to how many gallons you put into your tank.
Drive your car as you normally would until your gas tank is almost empty.
Note the mileage on your trip odometer. Refill the tank and note how many gallons you put in. Reset the trip odometer to zero. Do this for five fill-ups.
Add the miles covered and add the gallons purchased separately. Divide each of those totals by 5. This will give you the average miles traveled and the average gallons of gas purchased at each fill-up.
Divide the average miles by the average gallons and you will find the average miles per gallon your car is capable of. For example, if you averaged 213 miles between an average 8-gallon fill-up, then your car gets an average of 26.6 miles to the gallon (mpg). Your car will get better gas mileage if the majority of your driving is at highway speeds. By figuring the average during your normal driving, you will have a lower estimate than what your car is capable of on a highway. You want a lower estimate to account for traveling in cities and towns while on your trip and to allow for changes to mpg from the weight of luggage stowed.
Use an online mapping service or paper map to figure out the total miles you must travel to get from your departure address to your destination. Use the scale on the paper map to figure this out by measuring how many inches you must travel on the map and then converting it to miles (the scale will note how many miles are in an inch on your map). Enter in your departure address and destination address in the online mapping service you are using and click "Directions" and the service will automatically map the best route to take and give you the total miles to be driven.
Locate an online site that tracks the average national gas price per gallon (the link in the resources below is an excellent source). Add up the average gas prices for the states you will be traveling through, divide the total by the number of states you selected and this will give you the average price per gallon for your total trip.
Divide the total number of miles to be traveled by the average mpg of your car. This will tell you how many gallons of gas you will need to purchase for your trip. Multiply this amount by the average cost for gallon to find the expected cost of gas for your trip.
Add to your expected cost for gas the number of tolls you will encounter on your trip. You will find the tolls marked on your paper map or designated by your online mapping service. Call or go online to the state's highway department to find out the toll price.
Divide the total number of miles to be traveled by 65 (this is the average highway speed). This will give you a general idea of how many hours you must travel at 65 mph to cover your travel distance.
Divide the total number of hours needed to drive the distance on your trip by the number of hours per day you plan on driving. This will tell you how many days you will be on the road.
Set a budget for food and lodging that you can comfortably spend each day while traveling. Go online and search along your route (picking towns by the amount of distance you plan to cover in a day) and research the cost of lodging. Decide if you will be eating out or taking food with you in the car and plan a budget for food. Add your planned expenses for food and lodging for each day you will be traveling together.
Add 10 percent of your total food and lodging budget to the total to cover unexpected expenses (such as taxes).
Add the total cost of gas for your trip (plus tolls) to the total cost of food and lodging. This is the expected cost to drive on your trip.
