AvGas - Fill it up or wait?

[AlanA]

I routinely fly to two airports in my J. The trip takes about 30 gallons. At one airport the gas is about one dollar cheaper than the other airport. Here is my question: Is it cheaper to fill the tanks to the brim with cheaper gas but have less a less efficient flight with the unneeded full tanks? Or is it cheaper to get just the amount of gas needed at each airport regardless that the one airport is so much more expensive and have a more efficient flight. Or in other words, I have to add a little fuel no matter what at the expensive airport to have a reserve, but how much should I add at the cheap airport? Or still in other words, how much does it cost to carry around extra fuel that you don't need?

[Bolter]

The POH has charts on fuel burn and time to altitude versus weight. Looking at an extreme case, there is approx. a 1 gallon difference to altitude for 200 pounds more weight. This assumes a Vy climb to 12k MSL from sea level, but gives a sense of scale. I think real life does not follow this chart, but it should not be that different from the climb you really do and what fuel you burn. I believe a drawn out cruise climb is overall less fuel than the Vy to cruise altitude procedure.

So you are spending 1 $6 gallon of gas to save 30 gallons of $7 fuel. Seems to that is pays off, as long as you are not exceeding runway requirements at one airport to carry the cheaper fuel!

-dan

[Jeff_S]

I used to try to figure this stuff out. Before they put in a self-serve pump at KPDK, most of us would fly to outlying airports to get cheaper gas. I tried to figure out the relative costs vs miles flown and even had a little spreadsheet I tried to put together. Finally decided it was too much calculating and besides, the fun was in flying anyway.

[johnggreen]

Not meaning to denigrate your attention to detail, but the day I have to worry that much about fuel cost, I'll quit flying.

Jgreen

[1964-M20E]

I agree the fun is getting there. My normal procedure is to top of the tanks when ever I land. However, I have on occasion not fueled up at an airport if it was a short flight and I had plenty of fuel to get me home.

I will check local airports at my destination to find the best deal and go there if the driving penalty is not too great and the cost per gallon difference is. I’ve even looked at stopping 50 to 100 miles from my destination and filling up on cheaper gas but again the time penalty vs. the $ saved has to be there i.e. 30+ gallons needed and >$2 differential. I’ve never been on a trip where I needed more than 4 hours of flying to get there so I can do it in one hop without stopping for fuel.

[fantom]

While calculating be sure to include both the extra fuel burned during landing, taxi, takeoff, climb, and the additional wear on your plane.

[201er]

Guys, there is the hard way of figuring this out (spreadsheets, calculations, etc) and the easy way to solve this once and for all. Just see how many gallons it takes each way! If you don't have accurate fuel gauges or a fuel flow, you can do it using the pump. One time fly the flight half tanks (or whatever you normally do). Top it off on arrival, see how much that used. Fly back (assuming neutral winds) and top off again. Compare number of gallons on the pump. I'll be curious what you find.

I can tell you for a fact it pays for me to fly to an airport 10 minutes away to get gas and not at my field. 10 minutes at 10gph is 1.6 gallons and times 2 is 3.3 gallons round trip. We'll round up to 4 for run up and all other factors. $6.90 vs $5.54 justifies the cost of flying to get the fuel and I see it as free flying and a saving. Usually I pick up 80 gallons at a time $443 vs $552 at todays prices! That's $108 difference. Yet the cost in gas to fly back and forth for the gas is $22.16, $40 at most if you factor every nuance in. I know this doesn't attack your specific dilemma of flying heavy (and on my short flight it's irrelevant) but just to illustrate the huge difference in pricing.

In your case, you are saving $30 by topping cheap. I doubt it takes an extra 6 gallons to haul 30 gallons for 3 hours so without testing it, sounds like a good plan.

[jetdriven]

Dont forget the cost of another cycle on the airplane and .4 on the tach. You have a unique situation as you have 100 gallons. Ours only holds 54 so it rarely pays. However, our airport is run by a felon that we have had serious issues with, so we will fly somewhere else to prevent him from getting our money.

[AlanA]

Thanks for the calculations. I've wondered about that calculation for a while. Since it's just me, the gas tanks, and long runways at each airport I'll be filling up with the cheap gas for two reasons, (1) to save $$, and (2) because I hate getting reamed at the expensive airport.

[201er]

However, our airport is run by a felon that we have had serious issues with, so we will fly somewhere else to prevent him from getting our money.

Same here! What is it with airports being run by criminals? With all these security fears and whatnot, you'd think they would do a closer check of who is allowed to run an airport?

[danb35]

Well, 30 gal ~= 180 lb., or about the same as one pax. The cost difference is ~$30 (at $1/gal), which at today's fuel prices works out to ~5 gal. At my cruise power settings, 5 gal. lasts a bit over a half hour. Will 180 lb of extra weight slow you down enough to make your trip take 30 minutes longer? For me, that's about the same as a 25-kt headwind. Yes, the weight will slow you down, but nowhere near that much.

The situation gets more complicated when you're making an extra stop for cheap gas, but when you'd be stopping there anyway, buying at the cheaper place is the obvious answer.

[flyboy]

I would go with the cheaper gas. It does not cost that much extra to haul fuel. If you are pushing max gross, that's a different story.

[Mooneymite]

Not meaning to denigrate your attention to detail, but the day I have to worry that much about fuel cost, I'll quit flying.

Jgreen

Ha! Different strokes for different folks.

I love this part of flying: the most miles for the fewest bucks. Figuring wind components, delta-T, fuel cost, fixed cost, ramp/handling fees all part of the big equation for me. I'm a Mooniac, so price efficiency is all part of the fun.

I fly for a big company that plays the fuel price game to a huge degree. It can pressure FBO's to price competitively, or we fuel around them.

[phecksel]

how much is your time worth?

[Bob - S50]

I'd say in your case, fill her up!  Extra burn should be minimal.

 

I've come to the conclusion that if I need to stop for fuel enroute (say Seattle area Denver area), and there are multiple possible stops ... along the route ... and all other factors being equal (runway, approaches, food availability, etc), go to the cheapest one.  However, it is almost never worth the effort of making an unnecessary fuel stop just to buy cheap gas.  The extra fuel burned is the fuel used for: miles flown out of the way to get to the airport, pattern and landing, taxi in, start up, taxi out, takeoff, climb back up to altitude, and mileage to get back on course.  Unless you are just going a short distance and staying low anyway, you'll burn 1.5 to 3.5 gallons just for the climb.  Add the other stuff in and you are probably burning 5 gallons or more.  That's $30.  If you land short to get the cheap fuel, you probably aren't nearly empty so for most of us we can only take on 30 to 50 gallons.  You would have to save $0.60 to $1.00/gallon just to break even!  And now, your arrival at your destination is delayed by 30 minutes or so.  Not worth it in my opinion.  If I waste 30 minutes to save $10, that's not worth it to me (or my wife!).  And I'm a cheap pilot!

[Jamie]

Actually, I've been planning fuel stops to build a relationship with a particular FBO. It may or may not be cheaper, but I want them to get my business because I plan to stop there (the city) quite a bit. Buying gas is an easy way to do that.

 

I've also bought gas somewhere other than where I'm based because the the desk clerk was rude. It may have been cheaper to go elsewhere, but that wasn't anywhere in the calculation.

[garysuperpilot]

I seem to fly every week or two just to make a fuel run. Fuel at my home base 6.65 and there are four airports within fifty miles priced at 5.25 or less per gallon. I usually take the runs to give me and my bird a little exercise and have a little father and son time. What I save in fuel costs pays for my "little" joyrides to nearby airports. Any excuse to go flying seems like a good excuse to me.

[N201MKTurbo]

The extra weight has almost zero effect on cruise speed, it will make your climbs a little slower. For $1 a gallon, fill up at the cheaper airport.

[Bob - S50]

OK, here comes the geek in me.  First the formula, then the explanation and assumptions.

 

$/Gallon savings to break even = ((D/17 + (A * .14) + .8) * P)/N

 

Ouch !

 

D = Extra miles you fly out of your way to get the cheaper gas.

A = Altitude you will cruise at in thousands

P = Price of the fuel you buy

N = Number of gallons purchase

 

Assumptions:

Cruise economy = 17 nm/gal

Descent fuel saved = 0.1 gallons/1000' of descent

Pattern fuel burned = 4 nm @ 6 gph = .3 gal

Taxi fuel burned = 10 minutes at 2 gph = .33 gal

Takeoff fuel burned = 30 sec @ 18 gph = .15 gal

Climb fuel burn in excess of normal cruise burn = .24 gal/1000' of climb

Cruise altitude the same before and after the fuel stop

 

Example: Cruise at 3000' and you will fly directly over the airport you will stop at so zero extra miles, you buy 30 gallons and fuel is $6/gal:

((0/17 + (3 * .14) + .8) * 6)/30 = 0.244

You have to save $0.25/gallon to break even

 

Example: Cruise at 8000', fly 30 miles out of your way, buy 40 gallons at $6/gallon:

((30/17 + (8 * .14) + .8) * 6)/40 = .553

You have to save $0.55/gallon to break even