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MPG & Speed

In many cases, the best volumetric efficiency occurs at or near the torque peak (not the horsepower peak).
Putting along at 1100-1200 rpm in top gear is not in this range, and the engine would surely protest.
 
Agree On VE Importance

In many cases, the best volumetric efficiency occurs at or near the torque peak (not the horsepower peak).
Putting along at 1100-1200 rpm in top gear is not in this range, and the engine would surely protest.
I tend to agree, since both my Airhead and GoldWing get better MPG on slower, rural roads, in 2nd thru 4th gears.

Despite others' ideas above, I am not willing to seriously modify riding habits in order to squeeze a few more MPG out of any motorcycle. When time to fuel up, I am at the mercy of the pump, to include whatever the cost of a gallon happens to be. It would be nice if Airheads had fuel gauges though, other than the reserve valves.
 
My 81 R100RT would consistently give me 55 mpg at 60 miles per hour(100kph). When I would run at an indicated 120kph the fuel economy would drop to 45mpg. Both are imperial gallons. As for the reserve vs gauge.. in 40 years of riding my R100RT I have run out of fuel once (in sight of a service station) on the RT and knew I was going to be walking because of my poor fuel management. When riding to the Salt Lake MOA rally on a R1200GS sidecar rig I owned the fuel gauge read there was 24 km of range left and so, after deciding to ride to the next exit to a fuel stop I was familiar with in Helena MT, I was out of fuel with an indicated 18km of fuel still available on the fuel gauge. No walk was needed as I carried rotopaks on the rig. I still trust the reserves on my airheads and stop for fuel when I show two bars left on my R1150RT sidecar rig.... while still carrying extra fuel. :laugh
 
On my bikes, 45-50mph is the sweet spot. I can average as much as 58 mpg 5th gear at 51mph.

I'll bet that would be true for my bikes as well, but for me, that is a theoretical data point, much like making dietary decisions based on what I would weigh on the moon. Living where I do there are many places to give an Autobahn bread bike it's head. My K1200RS will run from 38mpg at a steady 100mph to as high as 55mpg at 65 to 70mph, or so I've read. :brow
 
Involved in best mileage at 50-60 in fifth gear ...

Lower aero resistance at lower speed

Lower engine rpm ... 50-60 is about as low rpm as you can go in fifth

Wider open throttle ... avoiding pumping losses.

Recall when VW Rabbits (and others) had an upshift light? This was to get you into a higher gear—and consequently wider open throttle—as soon as practical. You know of course that a major reason for diesel fuel economy is that they have no throttle. Some current or recent BMW cars control engine speed via variable valve lift for similar reason. There is no real control of vacuum effect on fuel delivery with carburetors ... as there can be with fuel injection. Current R1200RT, for example, blow away Airheads as regards fuel economy. They are bigger, heavier, faster and have more power ... and return better fuel economy. A lot of the Airhead loss of fuel economy is ... pollution, I.e. unburned fuel. FI takes care of that. Know the Bob Dylan description of fueling: carburetors know what your engine wants; fuel injection knows what it needs.

PS: no carburetor does it better than a constant velocity (CV) carb like the Bing. Still, covered wagon compared to fuel injection.
 
Current R1200RT, for example, blow away Airheads as regards fuel economy.

Over the years, I've noticed significant improvements in mileage from my Oiler, my Hexhead, and my Wethead over my old Airhead. The Hexhead's best number was 54 mp(U.S.)g; quite remarkable.
 
I've found that the mileage I get can vary by up to 5 mpg or more, depending on the blend of local gasoline. For example, I live in Boise, Idaho, and when I take a ride out of my area (150 to 200 miles) - either west into eastern Oregon or south east to the Salt Lake valley the fuel I buy there will give me a 3 to 5 mpg increase. On cross-county trips I've seen a lot of variation as well, so always watch my fuel gauge after every fill-up to get an idea of what kind of mileage the last gas I bought is giving me.
 
I've found that the mileage I get can vary by up to 5 mpg or more, depending on the blend of local gasoline. For example, I live in Boise, Idaho, and when I take a ride out of my area (150 to 200 miles) - either west into eastern Oregon or south east to the Salt Lake valley the fuel I buy there will give me a 3 to 5 mpg increase. On cross-county trips I've seen a lot of variation as well, so always watch my fuel gauge after every fill-up to get an idea of what kind of mileage the last gas I bought is giving me.

One variable often overlooked is altitude. I usually get better mileage at higher altitudes.
 
One variable often overlooked is altitude. I usually get better mileage at higher altitudes.

Interesting point. I've seen altitude/mileage changes with my K75's (even using the altitude plug - which I have connected to a switch), but the K1200RS doesn't seem to care much.
 
The altitude difference between Boise and Salt Lake City is only about 1,500 feet. I wouldn't think that would make a huge difference and gas mileage. Some but there must be other factors.
 
The altitude difference between Boise and Salt Lake City is only about 1,500 feet. I wouldn't think that would make a huge difference and gas mileage. Some but there must be other factors.


I've ruled altitude out as a factor, as I get the mileage boost consistently across different altitudes as long as my riding style and road conditions are similar. That GPS altitude feature is very handy.
 
I've ruled altitude out as a factor, as I get the mileage boost consistently across different altitudes as long as my riding style and road conditions are similar. That GPS altitude feature is very handy.

On our bikes Altitude is one of the data fields always displayed. An old habit from riding a K75 with an altitude "plug" which was actually a handlebar switch. Both here and in New Mexico when we had a condo there we went up and down enough to need to know when to switch the plug in or out.
 
On our bikes Altitude is one of the data fields always displayed. An old habit from riding a K75 with an altitude "plug" which was actually a handlebar switch. Both here and in New Mexico when we had a condo there we went up and down enough to need to know when to switch the plug in or out.

When I started traveling with a GPS I found the altitude display almost or more useful than the location/routing functions.
 
I've had the altitude plugs, switched, on my K's, switching them on and off at 5,000', but really never noticed a difference in mileage or performance. Something I haven't seen much discussion about here is the winds you encounter impacting mpg. I know from fighting a head wind or quartering cross wind heading west across the plains I've watched my mileage drop.

My '85 K100RS w/EML, generally gets around 35 mpg, running 70 mph, but in it's early years, it dropped to 17 mpg. That was definitely speed related, when my friend and builder Ozzie decided to take it to Mexico and race it in the La Carrera road race, in '87. He took the boat off and modified it with a road race boat. He and his son, his monkey, took first in the sidecar class, and most of the solo bikes. After Ozzie's passing, I asked his son what he remembered about the race. He said that they were never under 100 mph, much of the time over 120, passing solo bikes on the outside going around corners, and running the tank dry at 100 miles. They finished in eleventh, beating most of the roadracing solo bikes. Road racer Larry Coleman, racing a roadrace hack, commented that they went around us like a land rocket.

IMG_8315.jpg

Yep, speed related...

IMG_0783.jpg
 
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I've had the altitude plugs, switched, on my K's, switching them on and off at 5,000', but really never noticed a difference in mileage or performance. Something I haven't seen much discussion about here is the winds you encounter impacting mpg. I know from fighting a head wind or quartering cross wind heading west across the plains I've watched my mileage drop.

My '85 K100RS w/EML, generally gets around 35 mpg, running 70 mph, but in it's early years, it dropped to 17 mpg. That was definitely speed related, when my friend and builder Ozzie decided to take it to Mexico and race it in the La Carrera road race, in '87. He took the boat off and modified it with a road race boat. He and his son, his monkey took first in the sidecar class, and most of the solo bikes. After Ozzie's passing, I asked his son what he remembered about the race. He said that they were never under 100 mph, much of the time over 120, passing solo bikes on the outside going around corners, and running the tank dry at 100 miles. They finished in the top ten, beating all but nine of the roadracing solo bikes. Road racer Larry Coleman, racing a roadrace hack, commented that they went around us like a land rocket.

View attachment 82470

Yep, speed related...

View attachment 82471



:clap:clap:clap Excellent story! Thanks for sharing.
 
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This forum needs a LIKE button, especially for posts like the one above by 6322 !
 
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