Boat Motor Gas to Oil Ratios (The Unlimited Guides)

Have you ever been waiting in line to refuel your outboard engine and felt utterly stuck on the fuel to oil mixing ratio? I understand that finding just the right mixture can be complicated and sometimes frustrating, but I have designed this article to help you get it right without losing your cool.

Two-Stroke vs. Four-Stroke Engine

Two-Stroke vs. Four-Stroke Engine

Traditionally, small engines were all created equal- powered by a two-stroke engine that required a sometimes confusing fuel-to-oil mixture. But, as you can guess, four-stroke engines had been typically reserved for much larger applications.

Though this still holds in many cases, the marine motor industry’s technological advancements now produce extremely clean and efficient motors in either variety.

Two-Stroke Engine

Two-Stroke Engine

According to Power Equipment Direct, a “stroke refers to how many stages (cylinder/crankshaft movements) a combustion engine needs to complete to finish a power (working) stroke.”

So in the case of a two-stroke engine, it requires two cycles to complete one full revolution. In one revolution of a two-stroke engine, a lot is happening- compression creates ignition while power and exhaust are released.

Though you should always refer to the operating manual or engine label for clarification, you can easily pick out a two-cycle engine because it has one filling port for gasoline and oil.

For a two-stroke engine to function correctly, you must use the recommended marine-grade oil and adequately mix it with gasoline. Later in this article, I will include tips and tables to help you get the right mix.

Four-Stroke Engine

Four-Stroke Engine

More suitable for commercial and heavy-duty applications, a four-stroke marine motor is more similar to an automobile engine. These engines take four cycles to complete one power revolution.

Unlike their two-stroke counterparts, these larger engines have separate compartments for their oil. These larger engines also have significantly more moving parts than simpler-designed (but equally efficient) two-strokers.

Because a four-stroke engine can individualize each mechanical event throughout the combustion/power cycle, these engines separate the oil from the fuel (reduced CO2 emissions) and reduce unburned fuel release.

Do All Engines Require A Fuel-Oil Mixture?

Great question! But hopefully, you remember from the above discussion of two-stroke and four-stroke engines that only one of them requires a fuel-oil mixture.

The two-stroke (two-cycle) engine will need a manual mix of fuel and oil to ensure proper mechanical function because the oil lubricates the crankshaft and other moving parts.

Maybe you are new to boating or just acquired a boat by luck and need some assistance in even figuring out what kind of motor you have.

Let’s run through the various types of boat motors and discuss their fuel and oil requirements. Then in the following section, I will share several tips on how to mix these two essential fluids.

Diesel Inboard Engine

Diesel Inboard Engine

In Europe and most other parts of the world, automobiles and other vessels typically use diesel engines to power their crafts. However, in the United States, it is more common to see conventional gasoline engines.

Commercial boats or boats over 30 feet will utilize powerful diesel inboard engines in the marine environment. And in recent years, diesel engines are becoming cleaner and much more efficient than ever due to continuing improvements and regulations.

According to Rob Scanlan, an accredited Master Marine Surveyor, these engines must undergo compression testing during periodic tune-ups. Since diesel engines rely on high rates of compression to ignite the engine, any fluctuation could have a damaging effect on the internal parts of the engine and cause you to experience “a marine engine [that] is running rough or lacking power.”

Sterndrive Motors

Sterndrive Motors

A combination of inboard and outboard engine characteristics birthed the marine sterndrive motor. Typical inboard motors, a sterndrive motor will usually be a four-stroke engine.

The combination of inboard and outboard characteristics results in a four-stroke motor mounted at the boat’s stern just inside the transom. These motors have an extended steering shaft propelled by gears that extend through the bottom of the transom with its propeller aligned, similar to an outboard motor.

Typically the decision between a sterndrive or outboard motor comes down to preference. Usually, smaller vessels will opt for a sterndrive motor if they seek its following advantages:

  • An Unobstructed Stern– can be advantageous for you if easy access in and out of the boat is essential. In addition, this is a worthwhile consideration if you plan on using your boat for watersports. And, lastly, you have the option to install a rear deck since you do not have any motors in the way.
  • A Quieter Ride– can be achieved by utilizing specifically formatted soundproof material inside the engine bay. This feature may be exciting to those boat owners who do not like noise or refuse to speak above the engine while entertaining guests.
  • Aesthetic Appeal– though outboard motors are pretty sleek looking and tend to give an edgy appearance to the stern of one’s boat, many boat owners prefer not seeing their engines. With the location of a sterndrive at the boat’s stern and enclosed within a locker, there is almost no visual evidence of a motor onboard.

Inboard Engine

Inboard Engine

Like a diesel inboard engine design, you will find a conventional gasoline inboard motor towards the center of the boat’s hull alongside the transmission.

A drive shaft then runs the boat’s length to the stern, whereby a propeller drives the engine, and a rudder achieves a directional movement.

Whether marine-grade or converted automobile engines, these motors need significantly more mechanical skills to operate and maintain. These four-stroke engines can produce very high torque levels- dangerous and too heavy for smaller vessels.

Inboard engines are typical boat applications requiring significant power due to their intended use and weight loads.

Outboard Engine

Outboard Engine

The industry’s development of outboard engines is truly one to be marveled at. As the name suggests, an outboard motor is visible from the outside of the boat.

An outboard engine attaches to the boat’s transom, which is the rear-most cross-section of the boat. The gear case and propeller sit in the water during operation; however, these motors have a tilt function that lets you lift these parts out from the water during storage or when not in use.

Historically, outboard engines came standard as a two-stroke motor; however, modern-day manufacturers have developed two- and four-stroke designs.

Manufacturers offer a line-up of outboard motors designed to be rugged workhorses for your boat on today’s market. So, it would be helpful to decipher between the three significant types based on each of its fuel-oil requirements.

Conventional Two-Stroke Engine:

The frustrating, and often complicated, need to mix fuel and oil commonly refers to these conventional two-stroke outboard engines. Two-stroke motors range in size from 2.5 horsepower to over 500 horsepower, depending on the size of the boat.

So whether you prefer taking a Jon boat out for a relaxing day of exploration at the lake or plan to venture out into the deep blue, an outboard engine will do the trick.

And if you need more power than one engine can produce, add on another one, two, or three outboards to your transom, provided that the capacity of your boat will allow for it.

In any case, the more modern two-stroke engines will not necessarily require you to perform long-hand mathematical calculations while standing at the pump to refuel your engines to figure out how much oil to add for the proper mixture.

Manufacturers have designed two alternatives through innovation and development: the Direct Fuel Injection and an Electronic Fuel Injection outboard engine.

Direct Fuel Injection OB Engine:

When a two-stroke or four-stroke outboard uses direct fuel injection, these motors require the fuel and oil to be separated- a characteristic we already learned to pertain to the larger four-stroke design.

Fuel injectors release fuel into chambers that receive ignition from a spark plug. So like automobile engines, no priming is necessary, and owners can enjoy both a fast start and continued optimal engine performance.

Electronic Fuel Injection OB Engine:

Emitting very low emissions and excellent fuel consumption, electronically fuel-injected outboards achieve similar performance results as direct fuel injection designs.

With electronic fuel injection, there is a waiting period in which the fuel sits in the intake valve of the combustion system. In addition, an electronic unit controls the fuel injection timing upon the combustion chamber’s hottest point.

What’s The Mix?

What's The Mix

By now, you have a general idea of the differences between an inboard and outboard boat motor. But to maintain a reliable engine, you must understand how to care for its two most critical fluids.

Besides the mixture ratio itself, the process can become exponentially more complicated based on your location. For example, say you choose to take a coastal trip past the Canadian border; you will need a reliable conversion tool available to help you balance your engine’s needs based on their metric system.

But when stateside, most modern two-stroke engines “require a 50:1 [mixture] ratio”, but please do not overgeneralize this statement as it could negatively affect the performance of your motor.

Let me mention something you may not have thought about, especially if you are in the market for a brand new two-stroke outboard motor. New engines typically require a break-in period.

During this stage, believe it or not, the motor requires a richer content of oil, where you will implement a 25:1 ratio for this initial period. The purpose of this is to provide extra lubrication to the new moving parts of your engine.

Pro-Tip: Consult with your engine manufacturer upon purchasing your new outboard motor for any details or special considerations that impact its break-in period.

Tips On How To Find Your Motors Mix Ratio

  1. Visible Clues: Look all around your outboard motor for an inscription, sticker, or stamp that reveals the precise fuel-oil mixture ratio required for that particular model and type. Some common but less-known areas that you may find this information may be under the cowling, inside the fuel cap itself, or somewhere around the fuel tank.
  2. Handbook or Owners Manual: In older models, perhaps wear and tear have made such visible clues difficult, if not, impossible to make out. If you were fortunate to acquire the owner’s manual when you purchased your (used) boat, you would easily find the proper mix ratio within the owner’s manual. You will have access to this material that you should preserve in a dry and safe deposit somewhere on your boat for future reference with a new boat purchase.
  3. Online Search: Similar to the search you performed arriving at this article, you could perform a more specific search with the details of your outboard motor to garner professional advice or even a published technical narrative outlining your model’s requirements.
  4. Downloadable Mix Chart: Another great tool to use is to navigate your way to the manufacturer’s website. Often you will find a link to archived documents or tech bulletins where you can see, download, and even print specific mix charts for your model.

Mix Calculators

I’m old school, so I perform mathematical calculations with pen and paper, and that’s what I plan to impart to my boys as they reach the appropriate learning age.

However, when you need to fill up your outboards and get underway, using an online fuel to oil mix calculator is practical and convenient and a sure way to ensure you are feeding your outboard engine adequately.

My boating friends use this free Gas Oil Ratio Calculator to assist them with determining the proper amount of oil to mix with the amount of fuel according to manufacturer specifications.

The other fundamental advantage of utilizing this online tool is that it results in various measuring units depending on your location, including gallons and liters.

To make measuring as headache-free and straightforward as possible, you will find the oil amounts given to you in milliliters, pints, and ounces.


You do not have to let gas to oil ratios ruin any aspect of boat ownership. Please always remember to consult your motor’s manufacturer specifications and make sure you always produce your mixture in a separate and clean container- never directly into the engine.

The tools for finding the mix ratio required for your particular model motor and the online gas to oil ratio calculator should prove to be extremely helpful in maintaining your outboard motor.

If you need further clarification or assistance, please do not hesitate to leave your questions or concerns in the comment section below.

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