Motorcycle Fuel Mileage Issues

Warning: Controversial Material!

I get lots of inquiries about fuel mileage these days. It’s no surprise given highly volatile fuel prices! Here’s what I know about the subject.

(FILES) In a file picture taken on February 28, 2012 a motorist refuels at a gas pump in the northern city of Lille. French Prime Minister Jean-Marc Ayrault promised on August 22, 2012 a modest and temporary cut to the taxes on petrol and diesel in order to reduce pump prices for drivers. AFP PHOTO / PHILIPPE HUGUENPHILIPPE HUGUEN/AFP/GettyImages

Human nature tends toward exaggeration. This tendency is “encouraged” by the anonymity of the internet. Many reports I read online concerning fuel mileage on early ‘Wings can’t be taken too seriously. Some are merely “optimistic”…others are complete fabrications.

There are many factors that influence fuel mileage calculations including:

  • Vehicle engineering
  • Static weight of vehicle
  • Vehicle load
  • Rider technique
  • Apparent wind
  • Aero “sail”
  • Road gradient
  • Refill procedure
  • Odometer error
  • ANY reliance on the hopelessly inaccurate fuel gauge (avoid that!)
  • Weather (ambient temperature, relative humidity, barometric pressure, altitude, etc.)
  • Math errors (imperial gallons, liters, long division and such cause great confusion!)
  • Fuel quality (i.e., ethanol or other oxygenating percentage)
  • Etc.

Early ‘Wings were engineered in a time when performance was paramount. Just creeping into the equation at that time was the pesky problem of emissions control. Optimizing fuel mileage was surely not much of a concern to Honda engineers at the time. Fuel mileage was mainly the result of other decisions.

Back in 1976, I was in graduate school and did a research paper on automobile fuel mileage for a statistics class. The general thrust of the paper was to analyze many, many variables and determine what, if any, correlations existed between those variables and EPA official fuel mileage ratings. I considered a long list of obvious “mechanical” things like: engine displacement, number of cylinders, compression ratio, valve lift, cam timing, type of fuel system (carb vs. EFI), octane requirement, type of transmission, final drive ratio, length, height, width, wheelbase, overall “wet” weight, etc.

I also looked at “non-intuitive” variables like vehicle price, length of warranty, number of doors, country of origin, etc.

I won’t bore you with the details (statistics is like that!), but the only thing that mattered (statistically) in my study was vehicle weight. I’m not saying this was the definitive analysis on the matter …far from it. After all, my beer budget was rather large in those days! But, it proved to me that basic physics always prevail. Doing a certain amount of vehicular work requires a certain amount of fuel energy no matter what. If the vehicle is operating as engineered, there’s not much the operator can do (in normal driving) to affect fuel mileage in any significant way.

Expected Fuel Mileage for an early ‘Wing:

In my experience, anything around 35 mpg+ with acceptable performance and no operational anomalies like fouled plugs or hesitation is fine. Mileage claims exceeding mid-40s mpg are somewhat suspect.

Lower than expected fuel mileage points to many possibilities. These include:

Poor Compression

The mechanical inefficiency of poor compression can cause a drastic reduction in fuel mileage.

Respiratory Distress Issues

Dirty air filters or an internally blocked muffler / exhaust system cause fundamental “breathing” problems that rob power and efficiency. Ditto for improper valve lash adjustment.

Ignition Fault

Any number of ignition problems can reduce power on the affected cylinder(s). A wider throttle opening is required to compensate. A simple item like worn spark plugs can have a big negative impact on fuel mileage.

Carburetion Fault

Any type of carburetion problem that causes over-richness can reduce fuel mileage. See the chart here (written for GL1000, but most applies to any bike of similar vintage):

Brake / Driveline / Rolling Resistance Drag

Often overlooked, any excess brake drag or problems with low tire pressure, defective wheel bearings, universal joints, etc. can introduce parasitic losses and reduce gas mileage. Under-inflated tires are a chronic epidemic in my experience!

Clutch Slip:

A worn or poorly adjusted clutch can cause “slip” that reduces gas mileage. This is a very obvious and urgent fault, but some riders apparently are not mechanically inclined and don’t notice an extra 1000 rpms on their tach!

Poor Rider Technique

  • “Dragging” brakes while underway
  • Poor gear selection. “Lugging” in a gear that is too high strains the engine and reduces fuel mileage. Best advice: Keep the revs over 3000 rpms at all times. Save 5th gear for freeway speeds. Running the engine closer to the torque peak (approx. 6,500 rpms on early GL1000s!) is more efficient. These engines thrive on revs!

Other

Recommended Fuel

100% regular unleaded gasoline is the recommended fuel for early Wings. Adding ethanol to gasoline and marketing it as “gasoline” is unethical in my view. Avoid ethanol blends to the extent possible as they reduce fuel mileage. Click here for more information on ethanol. Premium grade is not necessary or desirable. This just wastes money and doesn’t provide any performance advantage.

All internal carburetor parts offered for sale by Randakk’s Cycle Shakk are approved for use with gasoline. The materials are long-established to be impervious to gasoline by the chemical firms which provide the materials. They will also tolerate a reasonable amount of ethanol in the “gasohol” fuel which is commonly marketed as gasoline in many markets in the US (with or without notice at the pump). E10 (10% ethanol) is an acceptable fuel as well as “seasonal” fuel (15% ethanol) which is marketed for summer use in some markets. This is the maximum recommended amount of ethanol in fuel marketed as “gasoline” for use in carbureted engines. Avoid ethanol altogether if you can, as ethanol attracts moisture which can lead to internal corrosion of metals inside the carbs.

Higher concentrations of ethanol are known to cause disintegration of rubber components. E85 (85% ethanol) must absolutely be avoided and not used under any circumstances…even emergencies. It will ruin the rubber fuel system components in any vehicle that is not certified as a Flex Fuel vehicle.

Continuous use of in-tank fuel cleaners or using in higher concentrations than recommended is bad and can lead to premature destruction of internal carb rubber parts. Float bowl gaskets in particular are prone to shrinkage in the face of fuel additive abuse. The only cleaner I recommend is the Yamaha product for “on bike” dirty carb rehab efforts…it’s effective and harder to misuse. Put the Yamaha product in the fuel bowls only…not in the fuel tank! More here.

Parting advice: “Pay the piper” at the gas pump and don’t obsess over your fuel mileage…worry about something else instead! 
Safety Tips:  
1. When refueling a motorcycle, always dismount before you begin dispensing fuel!
2. Take your helmet off if you intend to go inside the gas station to pay or use the bathroom. It’s the courteous thing to do for the clerks who work in these establishments. In many jurisdictions, it’s required by law.

Comments

  1. I ride a 77 Goldwing that I believe to be in better than average condition. I cannot afford a set of stock mufflers and have a set of Jardine mufflers on standard header pipes. Will the addition of the cross pipe of the stock muffler in any way enhance the performance (good or bad) on my bike.

  2. Adrian Campbell says:

    Hi I am wondering what happened to your electric fuel pump I do not see it on your site. I would like to install a electric pump on my 77 gl1000 if you do not sell one anymore can you recommend one that will work thank you and your site and info is amazing

    • Thanks for your interest.

      Our electric fuel pump conversion kits have been discontinued for a variety of business reasons:

      1. Supplier / sourcing issues
      2. Quality control problems not corrected by the pump manufacturer
      3. Pump intolerance of suspended rust particles which is now very common in these old motorcycles. Many customers insist on running with rusty, dirty fuel.
      4. Unwillingness of customers to follow installation instructions

      We may reintroduce an updated version at some point, Meanwhile, we recommend the OEM Honda pump as an appropriate replacement item.

Speak Your Mind

*