MPG to GPH calculation

Discussion in 'Flight Following' started by NHWannabe, Sep 11, 2020.

  1. NHWannabe

    NHWannabe Line Up and Wait

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    I just bought into a flying club that has a 1975 172M. The POH has all speeds in MPH and fuel performance is in MPH, anyone have a quick calculation to convert MPG to GPH? I'm suire it's fairly simple to come up with and the various EFB's do it in the background but I like to do it on paper every once in a while.
     
  2. EdFred

    EdFred Taxi to Parking

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    Well, yeah...

    flip the ratio to gallons per mile, then multiply how many miles you go in an hour.

    12mpg = 12m/g
    12m/g = 1g/12m
    1g/12m * 120m/h = 10g/h

    or mph/mpg
     
    Last edited: Sep 11, 2020
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  3. Ryanb

    Ryanb Touchdown! Greaser! PoA Supporter

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    It’s kind of an apples to oranges comparison, as it’s not always constant, but I figure it roughly like this...

    TAS - 120ktas

    GPH - 10.5

    Assuming zero wind, I can go 120nm on 10.5 GPH (average)

    so about 11.42 MPG.

    Our airplane engines aren’t very economical when you look at it like that.
     
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  4. RussR

    RussR Pattern Altitude

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    I don't know, what's the gas mileage of your car going 120 knots (138 mph)? If it can even go that fast, I'll bet the gas mileage is pretty low.
     
  5. MuseChaser

    MuseChaser Pattern Altitude

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    I did a lot of similar calculations and thinking when I started flying. MPG varies a lot with wind direction...a LOT..because of the huge variation in ground speed which is what affects mpg. Basically, our PA28 gets about the same mpg, on average, as our two big SUVs....but is twice as fast and can fly in a straight line instead of having to follow all those inefficient curves in the road resulting in less "miles" for the same distance between two points. Win for the plane. Of course, the SUV's annual inspection is $20....that's a big loss for the plane...;)
     
  6. Zeldman

    Zeldman Touchdown! Greaser!

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    I tried calculating the MPG of the race car once.

    It was not impressive to say the least....

    And considering race gas cost an outrageous $2.00 a gallon back then....
     
  7. iamtheari

    iamtheari Pattern Altitude

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    For mph and mpg, you can get gph by dividing the two. But probably the person who gave the mpg figure just calculated it based on mph and gph in the first place.

    As far as comparing flying to driving goes, you also have to consider the distance traveled and the cost of a gallon of fuel, just to get apples to oranges (without those, it’s more like apples to cabbages).

    For example, a 450nm flight I regularly take would be a 600-mile drive. I burn 9 gph at 130 KTAS, so with no wind I burn 31 gal. That’s 19 car miles per gallon of avgas. Of course, the avgas costs about twice as much as car gas, so it’s more like getting 10 mpg in the car. But I would have to drive an average of 170 mph the entire way to get there in the same 3.5 hours.

    In other words, you’ll never find a way to make this an apples-to-apples comparison.
     
  8. dmspilot

    dmspilot En-Route

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    That isn't right, there are much older Cessna POHs that show GPH. Are you looking at the right chart?
     
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  9. MauleSkinner

    MauleSkinner Final Approach

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    A whiz wheel makes it really easy.:D
     
  10. Dana

    Dana Pattern Altitude

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    It's elementary school arithmetic...
     
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  11. sourdough44

    sourdough44 Pattern Altitude

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    Flying is usually less than 1/2 the time of driving, and a lot more fun. Yeah, the usual factors to compare.
     
  12. chemgeek

    chemgeek Pattern Altitude

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    This is a simple units conversion problem:

    mi/hr * hr/gal = mi/gal

    So MPH/GPH = MPG

    My AA-5 gets about 16-19 MPG depending on cruise speed choices, better than some large SUVs. Of course, I'm burning fuel that's a lot more expensive...
     
  13. midwestpa24

    midwestpa24 Pattern Altitude

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    That was my question as well. I'm not sure I've ever seen a POH that shows MPG myself. Is this something the club created?
     
  14. NordicDave

    NordicDave Pattern Altitude PoA Supporter

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    Flying in a nearly straight line verses having to drive over and around to connecting freeways etc, saves a ton of time. The time is worth the extra money.

    Flying from Bay Area to Tahoe takes 1:15 mins or 5 hours driving, unless there is traffic which is then 7 hours.

    In others words, it's 230 miles by my car at 23 mpg is 10 gallons. The plane is 16 gallons at 12 GPH plus taxi.

    California auto gas is currently about $2.95 per gallon and av gas is $4.25 at my airport where I fill-up.

    Cost by plane = $68
    Cost by Car = $29.50

    I'll gladly spend the difference to save 4 hours of hassle. Round trip is like adding an extra vacation day. If I can buy a day off for $38 each way, done deal.
     
    Last edited: Sep 11, 2020
  15. NHWannabe

    NHWannabe Line Up and Wait

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    I probably should have read the entire Owner's Manual before posting because the charts in the back do have GPH. Since all the markings are in MPH when I came to this chart I assumed everything else would be in MPG.

    POH.JPG
     
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  16. midwestpa24

    midwestpa24 Pattern Altitude

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    Thanks for posting the photo, as it does indeed show MPG. MPG doesn't strike me as being particularly useful in an aircraft, because you will rarely have the conditions necessary for it to be a valid number. GPH much more useful. I even teach to not flight plan your fuel based on miles, but on time. For example when I take off in the Comanche, I have either 4 or 6 hours of fuel on board depending if I'm using the Aux tanks. In either 4 or 6 hours, I will be on the ground in one form or another. That may be 900 miles, that may be 300 miles, all depending on winds. Point being that fuel over distance is somewhat meaningless.
     
  17. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Actually, once you convert it to statute miles and you realize you're going three times faster than a car on the freeway, it's not that bad. The Navion gets about the same MPG my previous pickup truck got. My new RAM does a bit better.
     
  18. Racerx

    Racerx Pre-takeoff checklist

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    Try and buy some now. Last I checked it was above 5.00
     
  19. Dana

    Dana Pattern Altitude

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    You mean about the same as Avgas?
     
  20. Zeldman

    Zeldman Touchdown! Greaser!

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    It was up to 8 bucks per gallon at the track a few years ago. Of course buying by the 55 gallon drum it was a little less per gallon.
     
  21. SoCal RV Flyer

    SoCal RV Flyer En-Route

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    My EFIS makes the mpg calculation for me.

    RVs are pretty efficient...running lean of peak at a density altitude of 10,000', it gets about 23 mpg at 175 mph or so.
     
  22. Ed Haywood

    Ed Haywood Pre-takeoff checklist

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    MPG is not useful, because you do not have an odometer. You do, however, have a clock.
     
  23. Silvaire

    Silvaire En-Route

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    I guess that technically if you fly 100 mph for one hour you have flown through 100 miles of air. The fact that the air is moving relative to the ground is not Cessna's fault. :rolleyes:
     
  24. iamtheari

    iamtheari Pattern Altitude

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    It's really your fault for not buying the new Cessna with the Tailwind-O-Matic Wings.
     
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  25. EdFred

    EdFred Taxi to Parking

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    That option is only available on low wings.
     
  26. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    Well, all low wings except for the BE19/23/24 series, they all come with the Headwind-O-Matic wings.
     
  27. EdFred

    EdFred Taxi to Parking

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    I didn't say all low wings had that option available. :D
     
  28. frfly172

    frfly172 Touchdown! Greaser!

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    If you want to feel good about your mpg in the airplane. Get a boat and run those numbers, then you will really appreciate the plane.
     
  29. midwestpa24

    midwestpa24 Pattern Altitude

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    That is something I've always noticed. So many airplane owners will account for and try to justify the expense of owning and operating their airplanes. Yet others will own a $80,000 boat that costs a fortune to operate solely for pleasure use without batting an eye.
     
  30. Ed Haywood

    Ed Haywood Pre-takeoff checklist

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    The Cirrus automatically changes the wind direction of a headwind. If that fails, you pull the BRS.
     
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  31. RyanShort1

    RyanShort1 En-Route

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    Some planes do better... my old Taylorcraft would do 95mph all day burning 3.8 GPH.
     
  32. MuseChaser

    MuseChaser Pattern Altitude

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    Unless you get a sailboat..then you'll hate your plane's fuel usage....of course, you'll LOVE the plane's speed.
     
  33. MuseChaser

    MuseChaser Pattern Altitude

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    A bit different...can't recall a time when I thought, "Hey, let's go spend a week on the plane, cookin' and enjoyin' some nice meals, some bottles of wine and maybe the odd night cap, sleep beautifully with a nice breeze and stars all around, go to the bathroom whenever we have to,... ".... You get the idea. And that's on a sailboat worth about $5k. The world of power/speedboats is probably more in line with airplane costs and activities, but when you spend $80K on something that goes fast that you can't live on/in, you've already jumped the shark on practicality and ventured into the realm of "Because I like it and I want to!" Absolutely nothing wrong with that, either.
     
  34. EdFred

    EdFred Taxi to Parking

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    Yeah but to take 3 people somewhere you're gonna have 7 legs to get there (including bags) So your fuel burn is really an effective 26.6gph. :D
     
  35. RyanShort1

    RyanShort1 En-Route

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    Hmmm, but that's 3x the fun, and more logbook padding, too, FTW!
    For those of us that manage to keep our w... oh wait, that's not PC to say. ;) Suffice it to say, for solo missions, or two of my size people and a small amount of baggage, it's 21-23 mpg if the slowish speeds don't bother you - but it's only a small bit slower than a 172 and how many missions do many, many people fly solo!?
     
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  36. iamtheari

    iamtheari Pattern Altitude

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    You’ll also have an entirely different perspective on wind.