MPG or GPH

GrtLkBtr

GrtLkBtr

Member
Jan 31, 2007
34
St. Joseph, MI
Boat Info
1995 270 Sundancer
Engines
7.4L 300hp B1
While talking with a fellow boater, the subject of fuel consumption came up. I mentioned that I was pretty consistently getting 1.7 mpg. He suddenly scoffed “You cant accurately measure fuel consumption that way, you need to know your Gallons per Hour. MPG is only with cars”. He went on to explain that your fuel tank only has “time” in it began his rant of the relationship of gallons and time when operating a watercraft. It was during his gallons/space-time continuum explanation that I put on my best “that is interesting” face and my thoughts when to what chores that I had to do when I got home. His rant does raise a question that I would like to ask everyone, what is best… GPH or MPG? When I determined my MPG, I had downloaded all my miles traveled from my GPS. And as accurately as I could, subtracted all miles that were drifting with the engine off and divided that with the amount that I recorded when I fueled up a full tank. I have a database of nearly 4 years of this and the variance is only .2 mpg. I have even estimated at the time to fuel to the attendant that I should need 45 gallons or so and I was only 1.5 gallons off, petty good if I say myself. So when I say to someone “I get 1.7 mpg” does that indicate to the listener “this guy is not a real boater”? Or should I boast my knowledge and mention that I get 5 or so GPH??
 
IMO, do it any way you want and don't worry about what the other guys think. At some point or another he is breaking it down to -how far can I go on a gallon of fuel- even if he doesn't realize it.
 
cause i'm mentally challenged, I stop at GPH. If i'm feeling particularly sharp, I divide that by the number of gallons in my tank to estimate the time I need to start looking for a fuel dock.
 
Both have value, and neither on it's own constitutes the end-all be-all metric.

GPH can be used to determine how long you can go before you run out of fuel, and/or can give you an idea of the relative fuel efficiency of various RPM settings on your boat. MPG can help you determine how much fuel you will burn getting from point A to point B.

A real world example: my boat burns 40 GPH at 2400 RPM and 35 GPH at 2100 RPM. So 2100 RPM is more efficient using GPH, right? Not exactly, you have to factor in the relative speed and MPG.

At 2400 RPM, we go 26 knots (use either statute or nautical miles, doesn't matter, just be consistent). And at 2100 we go 22 knots. So at 26 knots and 40 GPH, that's 0.65 nMPG. At 22 knots and 35 GPH, that's 0.63 nMPG. So the higher GPH is actually more efficient. Yes, I realize that using the term "efficient" when discussing MPGs less than 1 is a tad ironic, but use whatever term you want... the math is the math.

Then you bring in the value of time. If I was making a 50 nautical mile run, the faster option would take 115 minutes and burn 76 gallons and the slower option would take 136 minutes and burn 79 gallons of fuel. So the faster option nets me 21 minutes and three gallons. So as I like to tell my wife, the more we run, the more we save.

No, by the way, she doesn't buy it.

And before Dom and the rest of trawler contingent start picking on my numbers, just remember that we're pushing a pretty heavy boat here. And all else being equal, I'd rather go faster.

Bottom line, neither metric alone is enough. Use them interchangably to support whatever bullsh*t story you need to tell to justify whatever you are trying to do.:grin:
 
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The fellow boater needs to go rant about something else.
I think most offshore guys or lengthy cruisers talk about nmpg. Fishing involves getting to point A, then B, etc. It's all translated to "how far can I run, and get home safely, on the amount of fuel I'm carrying". GPH means nothing; if you're burning 35 gph and doing forty knots you're better off than burning 25 gph and doing twenty knots. Point is you're going to burn less fuel getting from A to B.

With that said, people who just go out for a "boat ride"- no real destination in mind just out running the boat for a few hours- might only care about how much fuel they burn per hour of running time. That mentality never made much sense to me- most important is how far is can I get on a gallon of fuel.
 
The big variable that makes MPG a not so valuable number is sea conditions......wave heights, current, tides and wind. I find it much more useful to know time to a waypoint and GPH than to know MPG when the above variables can drastically affect miles traveled and efficiency of the hull and engines.

Of course if you boat on lakes and rivers and have dependable way to measure mileage, then sea conditions don't mean much.
 
Tim, Great explanation! I wish my fuel numbers were that good.

In my case my average fuel burn over last season netted an average of 30 gph. But at cruise speed I burn over 60gph. The reason for the difference is the great amount of time spend at lower speeds.

I agree that both are necessary. They are also very difficult to measure without real-time fuel flow meters.
 
I usually look at GPH at what my normal cruising speed is, which, if I wanted to calculate further would be MPG.

You have to use whatever you can frame it in your head the best.
We burn about 8 GPH, which is a little over 3 MPG.

I usually look at it as - hmmm... Saturday cost me $100 in gas, lol.

A boat hybrid would be nice :)
 
I've been planning a trip from Boston to Nantucket. I'm trying to estimate how much fuel I will use. I have no idea how many hours it will take to get me there and back. I can plot the distance with some accuracy. MPG works better for me to determing the fuel needed. GPH is useless in this instance.
 
Why does it really matter? If you are going on a 500 mile trip, then you need to use gph to know when you need to refuel and still have a good reserve when you pull up to the fuel dock. If your just putzing around, I use the fuel gage plus my intuition about how much fuel might be in the tank.
 
JuliaValentine said:
I've been planning a trip from Boston to Nantucket. I'm trying to estimate how much fuel I will use. I have no idea how many hours it will take to get me there and back. I can plot the distance with some accuracy. MPG works better for me to determing the fuel needed. GPH is useless in this instance.
Click to expand...

Boats don't work this way. You need to know GPH.
 
If you are of the opinion that GPH is useless, then you really need to go back and re-read my earlier post concerning why mpg isn't very helpful. If you are still of the opinion that GPH isn't important, then please leave a float plan with someone who isn't going with you on your trip.
 
Good gads. Repeat after me.

IT JUST DOESN'T MATTER

IT JUST DOESN'T MATTER

IT JUST DOESN'T MATTER

For most boaters, MPG and GPH are directly translatable. Now, Frank is definately correct: MPG if DRASTICALLY affected by sea condition. Of course, if you hit a storm -> it doesn't matter if you thing "MPG" or "GPH". Either way, you better have lots of fuel because your "GPH" will be high and your "MPH" will be low. F

or most boaters this is a non-issue, because 98% of us are not in the type of water where your MPG is affected (Afterall. . .how many of us are cruising offshore in a 48DB?). With real muck in the forcast, most of us landlubbers stay home.

- - - - -

I have a spreadsheet that calculates GPH. However, I know that in my boat, after eight hours of run time, I am looking for the fuel dock. Even if the *$#&% gauge reads 1/2.
 
but it's not quite that simple. I know that when I'm on a cruise, my MPH vary drastically depending on conditions and weight. MPG is a constant moving target.
 
Don't look at my first formula cause that won't work. I meant gallons in tank/GPH

See what I mean by challenged.
 
What's this term, MPG? I though it was GPM so you could stay away from fractions.
 
When I was planning my trip to Key West (Haven't gone yet), I had to take a look at MPG (or GPH with MPH) to determine how far E/NE I had to go out of my way to fuel up prior to turning SE to cross the Big Bend area. Depending on the boat's range, we have to go out of our way E/NE as much as 50 or 75 miles to cut 20 miles off of the crossing distance. I'm still not sure my boat can make the more direct cross from Apalach to Hudson (North of Tampa). As I look at it now, cruising from Apalach to anywhere across the bend is better than going NE to Carabelle to ensure the crossing to Tampa. I just don't know of anywhere to get gas between Steinhatchee and Hudson. There has to be some place, you would think. Cedar Key doesn't have any place to fuel up. It's very shallow in that area so any marina would have to have a channel cut.

When coming from Tampa, we fueled up at Tarpon Springs, cruised North to Cedar Key for the night, then crossed over to Steinhatchee for more gas. If it hadn't been for lack of sunlight, we may have crossed more directly, but not necessarily for the better. It's a pretty good stretch for our 340 (225 gallons, .85 - .90 MPG in calm seas).
 
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