Motorcycle Fuel Mileage Issues

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.

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:

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

• Fuel Leaks
• Anything else mentioned in this Tech Tip on Low Power Issues.

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.