Several years ago I was given an airplane to take care off while the owner was over seas for 3 year. This was a 1972 Cessna Cardinal. I had a little time in one and checked out with the owner before he left. The terms of the deal was fly it, keep it airworthy and send a bill at the end of the month. Cool!!!
I had the annual done and changed the oil every six months. I didn't fly it much, about an hour or two each month, mostly just around the local area. After having it about a year my sister in law had a wedding to go to in norther Minnesota. It was a long haul in a car so I offered to fly her in the Cardinal. It would even be a long haul in the Cardinal as well.
Not having a long cross country in the Cardinal, I got out the so called flight manual for the airplane. Even for 1972 it had pretty go planning information. After plotting my course on the Sectional (this was before iPads and cheap GPS), I looked up the power setting, TAS and fuel burn for my planned altitude. The morning of, I took the winds aloft and figure out my heading, ground speed, time en route and total fuel burn. I planned an hour reserve. Everything worked out to be a good non-stop flight.
So how many times have you heard that the book numbers aren't correct? Some say they're too conservative others say the exact opposite. So do you trust the book values? Do you go to the internet and see what's out there? Do you trust rules of thumb?
The internet is full of information. While some of it is good information, some of it is BS. So be careful of what you use there. Rules of Thumb can be good but you need to know the bases of these rules. Some times rules of thumb make you all thumbs and that ain't very useful. So that sends us back to the aircraft flight manual. At least that is a good place to start.
Well I flew that flight, all 4.5 hours of it. So how close do you think those book numbers came to reality? When I refueled I was less than a half a gallon off on the fuel burn and the flight time was right on the money! Folks this was a 35 year old airplane! So the book values can be reliable.
BUT! As Ronald Reagan said, "Trust but Verify". How many of you check your planning verses what really happened? When you land to check what you really burned compared to what you planned? I would bet most pilots don't. For all of my airplanes I keep a fuel log. I track the time and fuel burn. I can tell you my old 172 burned 8.8 gallons per hour at 75% power and my 79 182 burns 12.7 gallons per hour at 21 inches and 2200 RPM @ 133 KTAS.
I keep this fuel/time log to fine tune and double check my flight planning and to insure my airplane hasn't developed any fuel related mechanical issues.
Now I know some are flying helicopters or vintage aircraft where cross country flight planning information isn't available. Now what to do? It takes a little research and/or experimentation. Sometimes you can find power setting and fuel burn in the engine manual (found on the manufactures website or various book venders). For example if you look at the Robinson R44 POH it shows a optimum cruse speed of 100 knots which is right round 22 inches of MP. If you go to the Lycoming manual you can find the MP, RPM and percent power to come up with a fuel burn per hour.
Flight planning information for other aircraft can be a little more elusive. What I've done is developed my own power, airspeed and fuel burn chart. Pick a power setting and write down the airspeed and fly a few cross counties and keep track of the fuel burn. Do this for different power setting and at different altitudes. The key is to keep a log of your information! You eventually find a power setting, airspeed and fuel burn that you and the airplane like and you'll use that more than the others.
Having good, reliable information is one key on getting the most value and safety out of your aircraft. Don't just WAG (Wild, Ass Guess) it. You don't want to be one of those guys who flames out 5 miles from the airport due to fuel exhaustion!
Now go out and have fun!