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Thread: MPG formula

  1. #1
    jacuzer
    Does anybody have that MPG formula that someone posted awhile ago?
    Thanks

  2. #2
    LVjetboy
    Sure,
    The equation is pretty simple. You just have to know your hp at your target cruise speed, then:
    mpg = (12 x MPH)/HP
    If you know your pump/impeller specs, then you can estimate HP from RPM and the manufacturers pump curves.
    Cheers!
    jer

  3. #3
    jacuzer
    LVj,
    Thanks for the reply.
    Seems to me I read a formula that took into account the RPM and CID. (Mind you, I probably saw this late at night) but I remember doing a quick calc in my head so it must have been easy.
    Trying to figure out where my best mileage would be, get there faster and run the engine less, or don't push as hard and run the engine longer.
    I had to guess at this, running on Lake Mohave with not a gas pump to be seen.
    I'll try to dig up the graph on my pump and your formula should work.
    Thanks again

  4. #4
    Wet Dream
    Does 1.75 mpg sound right? 62mph (dash speedo) and 425hp?

  5. #5
    jroos
    I would like to know also. For OB`s its a gal. per hour at WOT for every 10 hp. Or if you have a 200, then 20 gph at WOT. I would love to know a formula for thrust/rpm/gph.

  6. #6
    LVjetboy
    Wet dream, if it cost you 425 to do 62, then 1.75 could be close. What's your engine, pump and boat specs?
    Jacuzzer and jros, I have a plot of mpg vs speed for some light weight tunnels, a shallow-v and a 21' Daytona...as calculated by JPC. Also have measured data for some other hulls, including a family deep v i/o for comparison. One trend you might find interesting....will post late tonight.
    jer
    [This message has been edited by LVjetboy (edited October 14, 2001).]

  7. #7
    Wet Dream
    LV, the engine is a 455 olds, G heads, wolverine cam (don't know specs) but its lopey, Holley 850DP, aftermarket intake manifold, dual plane, HEI ign, Bassetts OT headers, pump is JC12 with an A imp. The boat is a 19'Hallett model BV19. Low slung 6 passenger. I wish I had the weight specs and actual horses for you, I just working on a guess.

  8. #8
    kingb36
    Has anyone used the total milage counter on their GPS to check fuel milage like you do on a car? On second thought, I probably don't want to know!

  9. #9
    LVjetboy
    Jacuzer, here’s the mpg plot I promised with a 19’ tunnel, Shallow V, and 21’ Daytona tunnel:
    http://www.cedarcountyia.com/imghost/FuelChart.gif
    This data is predicted with the JPC (Fuel Tab) using the equation I listed above. The legend might be hard to read, so the blue is a 19’ PC tunnel, the purple an 18’ Advantage shallow V, and the red a 21’ Daytona. A couple interesting things. First, the Advantage was the only hull with data below plane speed, thus the hump. The overall shape of the curves can reveal a hull with a hook or other characteristics (carb, pump setup, etc.) that will either make for efficient running or a gas hog. The higher and more shallow the curve the better. Finally, notice how these hulls all had their best mpg just after plane speed. That’s something I really didn’t expect, so I toured the net for measured data to compare. Sure enough, that trend was often the case. Here’s a comparison with a Glastron GX 205, 20’ deep V with a 5.7 L V-8:
    Glastron (http://www.glastron.com/pages/BoatTest_GX205.html)
    Notice that measured fuel range of 5.2 to 2.8 mpg is within the range of numbers above. Also, the best fuel mileage happens shortly after planning.
    If you have your rpm versus speed data and would like to generate this chart, just go to the JPC page and bring up the Fuel Data Tab.
    JPC Page (http://www.angelfire.com/extreme2/dr...erfomance.html)
    There you can enter up to ten data points and have the calculator predict average or point mpg. I haven’t had time to check these numbers against actual data for my boat, but KingB36 has a good idea with the GPS…next time out I may just do that.
    Happy jetting!
    jer

  10. #10
    jacuzer
    This graph stuff is pretty cool, I think the plot is off quite a bit though for my boat. I burned 30 gal's (even) and made it about 60 miles on the weekend. Good water, so even hauling the gear I was able to cruise (and then a little skiing) used enough gas to run out just short of our cove. Looks like the place to run is just out of the water(assuming you don't have a gas pump handy)
    WD - compared to what I got on the weekend your 1.7? at 62 might not be too far off. I was trying to stay out of the throttle for the most part, that could account for the slightly better MPG's.
    JROOS - I want to know where I can get good water for an hour at WOT...Must be heaven... And the angels will even rebuild the old Ford when it blows!( or do they only rebuild Fords in Hell?)

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