What might cause a 10 mpg drop in my fuel mileage

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Turbojoe
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Joined: Fri Jun 03, 2011 10:35 pm
I ride: 2004 Vulcan 500
Location: Mesa, AZ

Re: What might cause a 10 mpg drop in my fuel mileage

Postby Turbojoe » Fri Nov 25, 2011 8:19 pm

I went ahead and did the oil change today. Put about 4 ounces of SeaFoam in the crankcase last night and let it run until the fan came on then let it "soak" overnight. Fired her up this morning and let her run again until the fan came on. Drained out the old Valvoline 20w50 MC oil. After almost 3,000 miles it still looked good. Nice amber color. Still I wanted to drop back to the 15w40 Rotella T because it's getting colder and I'm hoping to gain some gas mileage due to the lower viscosity. The shifting was noticeably smoother with the 20w50 but If I can pick up even a little mileage with the Rotella I can deal with the shifting difference.

The first oil change after I got the bike at around 6,400 miles I used a small amount of SeaFoam before the change and it came out black! The second change with SeaFoam added at around 9,400 and it came out "normal dirty" looking and the clutch started feeling better. This time at 12,200 with SeaFoam and the oil looks close to new. I'm sure the motor looks pristine inside now. For the buck or two worth of SeaFoam used on an oil change I'll keep using it each time. I've also been adding it to the gas no less than once a month and have no driveability problems. I love this stuff!

I pulled the plugs for a look see today as well and they still look like new. Didn't even need a re-gap. The air filter wasn't dirty enough to even bother washing (just washed and oiled it about 1k ago). The bike just runs great and I can't see anything service related that could affect my mileage. I'm absolutely convinced my mileage drop is related to the gas.

Joe
The nonsensical ravings of a lunatic mind.....

2004 Vulcan 500

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Turbojoe
Posts: 162
Joined: Fri Jun 03, 2011 10:35 pm
I ride: 2004 Vulcan 500
Location: Mesa, AZ

Re: What might cause a 10 mpg drop in my fuel mileage

Postby Turbojoe » Mon Dec 19, 2011 1:25 am

After the change from Valvoline 20W50 Motorcycle spec oil back to 15w40 Shell Rotella oil now that it has gotten cold here I noticed slightly rougher shifting but also picked up almost 3 MPG. Still nowhere the 50+MPG I saw during last summer. Plugs are good, I JUST serviced the air filter. I know the valve adjustment is good. I do have a few sets of new plugs and may go ahead and hit the valve adjustment again just to be sure. One way or another I want to find that 7 MPG that 'm still losing...

Joe
The nonsensical ravings of a lunatic mind.....

2004 Vulcan 500

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Triangles
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Re: What might cause a 10 mpg drop in my fuel mileage

Postby Triangles » Fri Dec 23, 2011 8:09 am

don't forget the often overlooked adjustment of your right hand. Some times mine gets out of adjustment and I see a 5-10mpg drop. Thankfully I usually get it back in adjustment before I see the red and blue disco lights behind me.
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Turbojoe
Posts: 162
Joined: Fri Jun 03, 2011 10:35 pm
I ride: 2004 Vulcan 500
Location: Mesa, AZ

Re: What might cause a 10 mpg drop in my fuel mileage

Postby Turbojoe » Sat Dec 24, 2011 3:09 pm

I'm pretty mellow on the bike. I never ride it like a sport bike. Never redlined at any time. 95% freeway riding at 70 mph. I've tried to ride 65 on the way to work at 3:00 pm but the cages will run me over. It's downright unsafe to ride the speed limit. On the way home at 12:30 am I've ridden 65 for a few full tanks and it made little difference in mileage. I've gone through the bike with a fine toothed comb. Everything tune related including cleaning the spark plug caps and cutting back the wires to new, clean wire. Carbs synched and pilots adjusted. Chain adjustment, tires pressures etc etc etc. Aside from a poor fitting intake boot the bike is in pristine condition. I'm still thinking it's the gas as it was such a sudden and consistent drop in mileage. I wish we could get some non alcoholic gas here in Arizona. I keep hearing people say that they've been upping the alky content for a winter blend. Probably just talk but I'm inclined to believe it at this point.

Joe
The nonsensical ravings of a lunatic mind.....

2004 Vulcan 500

Leaf
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I ride: '94 Maroon Vulcan EN500-A6, '05 Black Honda CMX250C

Re: What might cause a 10 mpg drop in my fuel mileage

Postby Leaf » Sat Dec 24, 2011 5:30 pm

I get a 42 mpg drop every time I get in the truck. >_>

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Turbojoe
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Joined: Fri Jun 03, 2011 10:35 pm
I ride: 2004 Vulcan 500
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Re: What might cause a 10 mpg drop in my fuel mileage

Postby Turbojoe » Sat Dec 24, 2011 7:24 pm

Leaf wrote:I get a 42 mpg drop every time I get in the truck. >_>


LOL. It's only a 20 or so drop for me to drive the car. Parking at work (and the gas mileage) are the biggest reasons the car sits for weeks at a time.

Joe
The nonsensical ravings of a lunatic mind.....

2004 Vulcan 500

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Triangles
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Re: What might cause a 10 mpg drop in my fuel mileage

Postby Triangles » Thu Dec 29, 2011 9:26 am

My experience with highway is that once you get over 65mph mpg's really drop off fast! One of my usual trips (about 150 mi) if I keep it near 65 I can easily make it in one tank. If I go 75+ I have to stop for gas before I get there. Note: all 3 Japanese bikes I've owned the speedo's read about 5-7% fast so that gauge 75mph is actually about 70mph. The odometer is also off by approximately the same.

I've changed my final gearing to a tall 38/17 (42/16 stock) 5th gear is now slightly taller than 6th used to be and 6th becomes like an overdrive. I only got half a tank on this before I parked my bike till spring due to the salting of the roads. We'll see how this new ratio affects my mileage in the spring.
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Leaf
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Joined: Tue Apr 19, 2011 6:54 am
I ride: '94 Maroon Vulcan EN500-A6, '05 Black Honda CMX250C

Re: What might cause a 10 mpg drop in my fuel mileage

Postby Leaf » Thu Dec 29, 2011 11:03 am

Yus, air resistance ends up being a complicated differential equation that I don't remember how to solve anymore....

Suffice to say, as you increase in speed, the air drag increases by some kind of nonlinear factor (it's exponential, but not terribly steep usually, unless you are driving my '84 F-150). There's a sweet spot around 45-50 mph with many vehicles where the point where the engine runs most efficiently intersects with a low area in the wind resistance curve where you get the best MPG.

Until you break the sound barrier, anyway, and then something totally weird happens. But I don't think we'll be doing that on the bikes (and /definitely/ not in the F-150). :3

Best way to get those MPGs back are to make the bike more aerodynamic (tucking in helps with this more than you'd think). The drag curve will still be there, but it'll open up a lot slower.


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