When towing, I estimate 10 mpg (diesel), and take that as a base-line for my costs. If traveling 1200 miles round trip, that would equal 120 gallons. 120 gallons multiplied by $2.00 a gallon would be $240.00 for fuel for my trip. I add in another 50% contingency for local running around and side trips. So that $240.00, add on another $360.00 for fuel costs for this imaginary trip. I then make sure I have the $360.00 available at the start of the trip and go have fun!
Simple, works for me. And when the end cost IS less than this, the remainder gets tucked away as a start (savings) for the next trip.
Local daily driving, fill it as it's needed. No problem.
Now that you have somewhat of a baseline ... about 11 mph, you can easily estimate the costs of your future trips.
