Fuel economy. Ever calculated yours?

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docmirror

Touchdown! Greaser!
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Cowboy - yeehah!
Well, I had a few long XCs to do, and wasn't in a hurry so I did some calcs of my fuel economy over the last weeks and the news is, well, not remarkable but interesting.

First leg, I used a normal TO, climb, and full throttle with 75% power and regular leaning method. At 6500' I got a dismal 12.2 MPG statute. Good news, I boogied along at around 188 SMPH.

Second leg, I went as frugal as I could, it was a long leg of 515 SM. I leaned on the ground, did a low power TO and climb, leaned in the climb, went up to 10,500, pulled back to ~50% power, ran LOP and generally did everything I could to minimize fuel use. I was headed into a slight cross wind that may have done little either way for my economy. Although I only blistered along at a paltry 153 SMPH, I got an amazing 20.6 SMPG!

So, next leg of 377 SM I tried a middle ground. Setup for 8500' with normal leaning and 65% power. This produced only 173 SMPH, but my economy was still dismal at 14 SMPG.

On the way back, I wondered if I could get the same good economy as on the trip out. Winds were similar, so this would partially make up for any gain I had on the second leg going west. I did all my economy tricks, took a long cruise section of 525 SM and went up to 11,500. I got 20.3 SMPG on that leg! Speeds were very similar at an average of 154 SMPH, or 134 Kts.

Now, this was with only one person on board, and very little baggage. I cleaned the plane well, and dressed the prop just before leaving. I'm sold on pulling the power back to ~55% and getting 20 miles per gallon. I actually saved a fuel stop on the way back, so that saved about 30 minutes that I would have spent if I had gone faster, but burned significantly more fuel. Another benefit, I don't have ANR headsets, but the noise level in the plane was much lower at the lower power settings. Oil consumption was abou the same, and CHTs were much cooler.

Anyone else done the same and got similar results? As I said, I wasn't surprised, but maybe it was a lesson that hit home because I kept such close track of the fuel burn. The only negative is that my engine 'worked' an extra 1.2 hours at the lower speeds, but it was lower RPM, so maybe that's not so bad either. As for me, the extra 1.2 hours wasn't a problem at all. Lower noise, stress, and easier on the turbulence too.
 
i tried, but that whole dividing by zero thing is tricky
 
yea i suppose it probably burns about 2 gallons on a typical tow. if you start to factor in the efficiency of the car during a long retrieve though the miles per gallon start to go way down!
 
How did you calculate your fuel mileage? Was it miles flown divided by fuel pumped into the plane, or MPH divided by fuel burn? I would think that normal variations in the filling of the tanks could make a significant difference in the calculated MPG. I've seen enthusiastic line folks able to squeeze another 4 or 5 gallons more into the tanks than others who conservatively fill the tanks because they have had to clean up too many spills from overfilling planes before. That can make a significant difference in MPG.

Good experiment, but I would encourage repeating the experiment several times before coming to solid conclusions.
 
Fuel economy is a normal part of every flight. The Shadin displays it in real time. I figure on 10 nmpg average -- more with a tailwind, less with a headwind.
 
Ken Ibold said:
Fuel economy is a normal part of every flight. The Shadin displays it in real time. I figure on 10 nmpg average -- more with a tailwind, less with a headwind.
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Our JPI with fuel flow shows MPG, but I haven't quite gotten the operation down.

Last night, training flight to TYS, 5500msl, 23", 2400rpm, about 155kts over the ground, fuel flow showing 11gpm (it was hot, and I wanted to keep it a little rich), shows about 16smpg.

Up high, I'd expect the mpg to increase to 18smpg or so.
 
flying from san diego to boston in an sr22, we did some fuel calculations, and figured that at 65% instead of 75% we'd save around $800 in fuel cost while only going 10-15kts slower. We never figured out the MPG. It was 12.5 gph at 165 kts.
 
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Hmm.. My last 'long' XC was coming back from Santa Fe last March. I was at 13,000' MSL, burning 7.7GPH with an avg 185 kt groundspeed. It took us about 4 hrs to get from Santa Fe, NM to Boone, IA so that means our rough average MPG was: 24nMPG. Is that right!? That's a very rough estimate.

I think I did some quick and dirty math one day while flying to Alabama. Seems like I came up with ~16nMPG no-wind average for the RV at 'normal' cruise of 2350/23.5" @ 10GPH. Haven't done the math for running LOP, though.

One thing that would be hard to calculate into the mix would be the decrease in ground covered for flying vs. driving. Not to mention if you factor in 'time away from work' for travel via plane as opposed to driving. The actual 'cost per mile' has got to be lower than driving (if you know how to work the numbers right. ;))
 
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Uh, might need to recalibrate that fuel flow.....:rofl:

Bill Jennings said:
Our JPI with fuel flow shows MPG, but I haven't quite gotten the operation down.

Last night, training flight to TYS, 5500msl, 23", 2400rpm, about 155kts over the ground, fuel flow showing 11gpm (it was hot, and I wanted to keep it a little rich), shows about 16smpg.

Up high, I'd expect the mpg to increase to 18smpg or so.
Click to expand...
 
Bill Jennings said:
Our JPI with fuel flow shows MPG, but I haven't quite gotten the operation down.

Last night, training flight to TYS, 5500msl, 23", 2400rpm, about 155kts over the ground, fuel flow showing 11gpm (it was hot, and I wanted to keep it a little rich), shows about 16smpg.

Up high, I'd expect the mpg to increase to 18smpg or so.
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Steve said:
Uh, might need to recalibrate that fuel flow.....:rofl:
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Geez, Steve. Didn't you know that they got the turbine conversion on their Mooney? Only problem is that they had to remove the rear seats for extra Jet-A storage. :D
 
NC Pilot said:
How did you calculate your fuel mileage? Was it miles flown divided by fuel pumped into the plane, or MPH divided by fuel burn? I would think that normal variations in the filling of the tanks could make a significant difference in the calculated MPG. I've seen enthusiastic line folks able to squeeze another 4 or 5 gallons more into the tanks than others who conservatively fill the tanks because they have had to clean up too many spills from overfilling planes before. That can make a significant difference in MPG.

Good experiment, but I would encourage repeating the experiment several times before coming to solid conclusions.
Click to expand...

I did miles covered div fuel pumped. I always pump my own fuel, so I know what went in is right. The bladders are 57 years old so I think any expansion is pretty well done by now. :p

I agree, I'll need to do some more flights and keep accurate logs before getting too excited. That last 25 MPH is very costly in most planes.
 
deafsound said:
flying from san diego to boston in an sr22, we did some fuel calculations, and figured that at 65% instead of 75% we'd save around $800 in fuel cost while only going 10-15kts slower. We never figured out the MPG. It was 12.5 gph at 165 kts.
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That's 15.18 SMPG which is pretty sweet at that speed. I think I could maybe get mine up to ~15 with better leaning. Now I have a EGT so I can lean better. That extra 15 kts just isn't worth $800 to me. I know it took longer, but it's worth it in my book.
 
NC Pilot said:
I've seen enthusiastic line folks able to squeeze another 4 or 5 gallons more into the tanks than others who conservatively fill the tanks because they have had to clean up too many spills from overfilling planes before.
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A gallon spilled is a gallon sold...
 
I did that calculation once in the Baron. Compared to the family truckster, it was pitiful. Until I ran the family truckster up to 100 mph and looked at the instantaneous mpg--it was around 6-8 mpg.

Now, considering I'm cruising in the Baron at 180-200 mph, there is no way the family truckster will meet my fuel economy. So, I'm happy once again.

Seriously, however, I don't think it is very helpful to compare such fuel economies of our planes vs. our automobiles. Obviously, if we want to seek the "best economy", we would all be riding bikes. :)
 
I get about 13.3 MPG in the Cherokee 180.

110 smph x 1.15 = 126.5 mph / 9.5 gph = 13.3 mpg
 
kevin47881 said:
I get about 13.3 MPG in the Cherokee 180.

110 smph x 1.15 = 126.5 mph / 9.5 gph = 13.3 mpg
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I think you meant "110 KMPH x 1.15 = 126.5......" Usually smph indicates statute, but you don't cruise that slow.
 
Whether in a car or a small plane, fuel consumption always increases disproportionally with speed...no? And boats most of all, unless you have a hydrofoil...

To squeeze that last little bit of book cruise speed out of any vehicle you are going to pay a disproportional price in fuel economy.
 
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No prob. I love the change from Knots to MPH. 'Nautical' has no K, but Knots without a 'K' is just not cool.
 
Last week: 863 NM CYFO-KISN-KGXY used 67.2 gallons. 14.8 mpg. Not too bad.
 
Last time I did the calc I got 13MPG for the trip. Not too bad. I care more about eh fuel burn/hour and calculate that every flight. I average just under 8gph
 
Why try to justify based on fuel economy? It might not look terrible, but getting there cheap isn't the point of GA.

I just flew a roundtrip to Salinas and back from Fullerton. Up one morning, back the next. Only averaged about 10 smpg (headwinds both ways, my usual curse). My flight time - IFR - was only about 4.7 hrs roundtrip, including instrument approaches in actual on both flights, vs. at least 12 hours roundtrip driving. Even including preflight/postflight times, I created over 6 hours of productive time in a two day period vs driving.

If I had flown commercial into Monterey, it would have looked like this:
Drive to airport: 45 minutes (vs. 5 minute drive to Fullerton airport from my house)
Arrive at airport 90 min before flight to park car, clear security.
Flight time including taxi time: 1.5hrs (I'm being generous to the airlines on this one)
Get rental car at destination: 20 minutes (The FBO pulled up my reserved rental car right behind my plane about 30 seconds after I shut down. They also pulled the fuel truck up immediately and fueled the plane while I was gathering my things and doing paperwork for the rental car-which they also brought out to the plane.)
Repeat for the return flight, total time for commercial: >8 hrs. And no flexibility on time, so I probably would have spent another couple hours, at least, waiting for a flight.

Oh yes, and by flying myself I was able to get back to the office in time to finish a $50K+ proposal and get it submitted before a 4pm deadline. This wouldn't have happened flying commercial or driving.

The cheapest possible cost for flying commercial would have been nearly as much as the cost of the avgas for the flight. A no-change-penalty airline ticket would have cost over $100 more than the fully-burdened cost of the GA flight.

Yes, a Cessna 182 is a time machine!

Jeff
 
For another datapoint, the cargo ship I was on in the USNavy managed 55 gallons per nautical mile. This was at 0 MSL and at 12,000 tons MGW. :yes: :D
 
Jeff Oslick said:
Yes, a Cessna 182 is a time machine!
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Not quite the speed of light, but very comfortable anyway. ;)

Our 182 burns just a hair shy of 13 gallons per tach hour. I plan on 130 knots, so that's 10nmpg. I'll have to get my car up to 150mph and see what it gets! :hairraise:
 
Jeff Oslick said:
Why try to justify based on fuel economy? It might not look terrible, but getting there cheap isn't the point of GA.



Jeff
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My reason for the calcs was the the comparative economy at different power settings and also a comparator to the driving cost in fuel. I had to go on these trips anyway, and I tried to maximize my fuel economy.

What I learned(and empirically knew anyway) is that the last 20-25MPH cost a heck of a lot of money. Mst of my trips now are going to be in the 150-160 MPH range, rather than previous speeds in the 180 range. I was always happy about the speed of the plane, but was disappointed with the fuel burn.

My truck gets about 17MPG at freeway speeds, and I have to follow the highway system. My plane goes approx 2X the speed, I get to go more or less direct and I can fly in comfort unless the weather closes in. That was my only point in this.
 
15.6 MPG using miles if I had made the trip by car divided by gallons I burned in the plane for a trip I made yesterday. It would have been 640 miles of driving vs 440 miles of flying so using straight line miles, I only got 10.75 mpg. Cherokee 140 (160)
 
It'd be interesting to find out fuel economy at the same speed you travel in your car.

I haven't tried this, but I would think that running at peak, at the optimum altitude, you could get up into the mid 20s.

-Felix
 
sba55 said:
It'd be interesting to find out fuel economy at the same speed you travel in your car.

I haven't tried this, but I would think that running at peak, at the optimum altitude, you could get up into the mid 20s.

-Felix
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Well, I'm not an engineer, but I'd think at some point the efficiency curve would deflect downwards on most small single engine planes below, say, 80-90 knots. So I'd bet your fuel economy in terms of nmpg would be pretty bad at those low speeds.
 
sba55 said:
It'd be interesting to find out fuel economy at the same speed you travel in your car.

I haven't tried this, but I would think that running at peak, at the optimum altitude, you could get up into the mid 20s.
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I'd figure your best economy might be at best glide with the power on, I'd have to try.
 
docmirror said:
That's 15.18 SMPG which is pretty sweet at that speed. I think I could maybe get mine up to ~15 with better leaning. Now I have a EGT so I can lean better. That extra 15 kts just isn't worth $800 to me. I know it took longer, but it's worth it in my book.
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Coming home from Currituck Co Sunday, we were flying 155 true, GPS showing 153-163 over the ground depending, 10gph, and the JPI was showing 15.8nmpg. Converting, that is about 18smpg, pretty darned good.

My F-150 Supercrew 4x4 gets 16mpg at 80mph on the highway, gas local is $2.70ish, so $0.169/mi. The Mooney was burning $3.75 av gas (decent price at Currituck), so that is $0.208/mi, so it cost me about $0.04/mi extra to fly.

Not shabby.
 
Those Mooney's are darn fuel efficient. I was amazed that the old Bonanza was as efficient as it was. At around 155Kts I will be at about 13 SMPG. I've gotta slow down to 140 before I get near 18.

I serached for the M20J models too, but couldn't find a decent one under about $80k. That was out of my price range, so ended up with a Bo. I found some F models that were lower, but didn't have the performance of the J.
 
Bill Jennings said:
I'd figure your best economy might be at best glide with the power on, I'd have to try.
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we figured this in one of my classes. its a tad faster than best glide for a piston powered airplane. has something to do with the propeller efficiency. but best glide is pretty close.
 
vontresc said:
Heh I'd love to see you do 150 in that Volvo of yours:goofy:
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I have no doubt it'll do 150. I once broke 100 in it by accident. :eek: :goofy:

Now, to find a stretch of road where I could get it that high relatively safely without getting caught is a different story. :(
 
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