MPG to GPH calculation

NHWannabe

NHWannabe

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NHWannabe
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.
 
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
 
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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.
 
Ryanb said:
Our airplane engines aren’t very economical when you look at it like that.
Click to expand...

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.
 
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...;)
 
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....
 
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.
 
NHWannabe said:
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.
Click to expand...

That isn't right, there are much older Cessna POHs that show GPH. Are you looking at the right chart?
 
Flying is usually less than 1/2 the time of driving, and a lot more fun. Yeah, the usual factors to compare.
 
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...
 
dmspilot said:
That isn't right, there are much older Cessna POHs that show GPH. Are you looking at the right chart?
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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?
 
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.
 
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dmspilot said:
That isn't right, there are much older Cessna POHs that show GPH. Are you looking at the right chart?
Click to expand...

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
 
NHWannabe said:
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.

View attachment 89851
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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.
 
Ryanb said:
Our airplane engines aren’t very economical when you look at it like that.
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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.
 
Zeldman said:
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....
Click to expand...

Try and buy some now. Last I checked it was above 5.00
 
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.
 
MPG is not useful, because you do not have an odometer. You do, however, have a clock.
 
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:
 
Silvaire said:
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.
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It's really your fault for not buying the new Cessna with the Tailwind-O-Matic Wings.
 
frfly172 said:
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.
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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.
 
Some planes do better... my old Taylorcraft would do 95mph all day burning 3.8 GPH.
 
frfly172 said:
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.
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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.
 
midwestpa24 said:
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.
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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.
 
RyanShort1 said:
Some planes do better... my old Taylorcraft would do 95mph all day burning 3.8 GPH.
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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
 
EdFred said:
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
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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!?
 
MuseChaser said:
Unless you get a sailboat..then you'll hate your plane's fuel usage....of course, you'll LOVE the plane's speed.
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You’ll also have an entirely different perspective on wind.
 
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