gas mileage question

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gas mileage question

Post by nlovern » Tue Aug 26, 2014 11:40 am

Hi everyone,
Nathan Lovern here. I had a gas mileage question. sat I went on short trip about 80 miles. it was misty n foggy with drizzle. I filled bike up to an inch of cap, gas gauge read past full.
Today I went on a short ride about 60 miles and the gas gauge said full still. I figured it was stuck. I went to put gas in it ( 90 oct ethanol free ) and it only held $2.68 to fill it to inch of top again.
On trip today it was dry sunny and low humidity. Took almost 1/3 tank. Gas gauge went down.
Trip on sat was open road, 5 miles up mountain, town and about 15 miles interstate then back on secondary road home.
Ive heard that wet rainy days engines get better mileage but I cant explain this wet ride mileage????
Anyone else experienced this???
Nathan Lovern

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Re: gas mileage question

Post by HawkeyeGL1200 » Tue Aug 26, 2014 2:42 pm

Are you certain no one else is adding fuel to your tank, and playing a practical joke on you?
I am wrong as often as I am right concerning what is wrong with someone else' motorcycle without having seen the machine in person. Guessing with limited information, as to the source of the trouble, is sketchy at best.

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Re: gas mileage question

Post by nlovern » Tue Aug 26, 2014 2:55 pm

I was wondering same thing. Next rainy day I'll try same trip again,see it I can repeat it

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Re: gas mileage question

Post by mrtwowheel » Tue Aug 26, 2014 6:20 pm

I think that the shape of these tanks sometimes trap air in the corners, especially when you fill while on the sidestand, other times the air pocket burps and fills with gas. Try judging your full tank when filling with the bike on the centerstand. As far as the fuel guage goes, mine are only accurate when on empty, just like aircraft.


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Re: gas mileage question

Post by Corkster52 » Sun Aug 31, 2014 7:33 am

I may not be doing it correctly, but to make sure I get the same amount of gas in the tank each time I fill it until it stops bubbling all the way to the top of the fill neck (can't get another drop in it).

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Re: gas mileage question

Post by MikeB » Sun Aug 31, 2014 10:52 am

Following the owners manual is what most would suggest.
Page from owners manual:

Tacoma, WA, USA

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Re: gas mileage question

Post by rachester67 » Mon Sep 01, 2014 2:15 pm

I'm no scientist but when its raining is generally cooler and cooler air allows for better fuel and air mixtures which could be the difference. I know back in the day people out on the Bonneville flats would use dry ice on the intakes trying to get better mixtures and faster times.

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Re: gas mileage question

Post by robb » Tue Sep 02, 2014 8:53 pm

I always fill up the night before or very early in morning before a trip. When temp is cooler the gas will condense allowing more fuel, then when it warms up start to expand. The heat of engine will cause expansion of fuel in tank and could account for why you could only get a small amount to refill. Will also give a false sense of great fuel mileage.

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Re: gas mileage question

Post by PastoT » Fri Nov 07, 2014 5:45 pm

Carbureted engines see a nice boost in cold humid or "wet" air. One cc of water expands to 1600 cc when vaporized, and that is many times more than the expansion of burn gasoline ( I forget the value of vapor from liquid for gasoline ). If we could burn water the efficiency would be incredible, alas that's a dream. Still the boost to an older engine running hot enough to vaporize water droplets is substantial, so much so that foggy or high humidity air produces an increase alone. My 750 Yamaha loved it also, so much so that in engineering school we built a water misting system into the carburetor intakes and injected a very minute and controlled mist of water once the engine was running up to temp. I was getting 100+ miles a gallon across Texas into Arizona; granted the engine ran very hot naturally there and was air cooled. Our bigger water cooled and EFI engines don't respond the same way, I was considering injecting my GL1800 a while ago but have other priorities now. You also mentioned Ethanol Free fuel... there's 3-5 additional miles per gallon for my 1800, and you were in the mountains. I've read that mileage is better in the mountains and so far my mpg agrees. Each time I'm above 5000 feet in the Idaho mountains my mileage increases (two up with 500# trailer and I was getting 40mpg vise my normal 35 alone with a trailer). This has something to do with air density and drag I suspect. My worst mileage is on E fuel near sea level - almost a cage worthy mpg!
Tom, in Mountain Home, Idaho
2002 GL1800 (Illusion Red) Non-ABS, 122k miles
Retired Air Force

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Re: gas mileage question

Post by WingAdmin » Fri Nov 07, 2014 6:52 pm

Yup, water injection was used in military piston planes in WWII for emergency or takeoff power. It's also used by dragsters. Water droplets evaporated by a hot engine (or intake, particularly in a supercharger) suck up heat energy, cooling the intake air, making it much more dense, so it can take on more fuel.

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Re: gas mileage question

Post by joeincalif » Tue Nov 25, 2014 9:54 am

You probably ride slower in the rain also. Slower speed equals better MPG'S
I usually get around 40 MPG. when I run over 3000 RPM'S or over a constant 70 MPH it drops way down. When I was up in high altitudes in Colorado I got a constant 50 MPG.


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