Gasoline vs Biodiesel: Vehicle and Engine Compatibility

The Short Answer

Gasoline and biodiesel are not alternatives for the same vehicle. Biodiesel is made for diesel engines, and gasoline is made for gasoline engines. A gasoline car cannot run on biodiesel in any blend. A diesel vehicle may be able to use biodiesel, but only up to the blend level its manufacturer approves. So the useful question is not which fuel is better. It is which engine you have and what that engine is approved to burn.

If you drive a gasoline vehicle and want a renewable fuel, look at ethanol blends instead. Most US gasoline already contains some ethanol, and flex-fuel vehicles can use higher ethanol blends.

Why the Two Fuels Cannot Be Swapped

The two fuels suit two different ways of burning fuel. A gasoline engine is a spark-ignition engine. It mixes fuel with air, compresses the mixture moderately, and a spark plug ignites it. Gasoline is volatile, which means it evaporates easily, and it resists igniting under compression alone. That resistance is what the octane rating measures.

A diesel engine is a compression-ignition engine. It compresses air until the air is very hot, then injects fuel, which ignites from that heat without a spark. Diesel fuel and biodiesel are heavier and oilier, and they ignite readily under heat and pressure. That property is measured by the cetane rating. Diesel fuel also lubricates the high-pressure fuel pump and injectors as it passes through them.

Each fuel has the opposite of what the other engine needs. Biodiesel will not vaporize and burn properly from a spark. Gasoline in a diesel system ignites at the wrong time and strips away the lubrication the fuel system depends on.

What Biodiesel Is

Biodiesel is a renewable fuel made from vegetable oils, animal fats or recycled cooking grease. These oils are chemically processed into a fuel that behaves much like petroleum diesel. It is not the same as straight vegetable oil or used fryer oil poured into a tank. Raw oils are not biodiesel and need dedicated vehicle conversions.

Biodiesel is usually sold blended with petroleum diesel. The number after the letter B is the percentage of biodiesel in the blend.

B100: pure biodiesel, used mainly as a blending stock and only in equipment specifically approved for it. B20: 20 percent biodiesel and 80 percent petroleum diesel, a common blend for fleets and many newer diesel vehicles. B5 and lower: small blends that can be sold as ordinary diesel fuel in the US under the standard diesel fuel specification, often without being marked separately at the pump. B6 to B20: covered by a separate fuel specification, and pumps are generally labeled to show the blend.

Gasoline and Biodiesel Compared

Feature | Gasoline | Biodiesel; Engine type | Spark-ignition (gasoline) engines | Compression-ignition (diesel) engines; Source | Refined from crude oil | Made from vegetable oils, animal fats or recycled grease; Ignition quality rating | Octane | Cetane; Typical renewable blending partner | Ethanol (E10, E15, E85) | Petroleum diesel (B5, B20); Lubricity for fuel system | Low; gasoline systems are designed for this | High; can improve diesel fuel lubricity; Cold-weather behavior | Flows freely in cold conditions | Can thicken or gel in cold weather, more so at higher blends; Use in the other engine type | Damaging in a diesel engine | Will not run a gasoline engine

Which Vehicles Can Use Biodiesel

Nearly any diesel vehicle sold in the US can use the low blends found in ordinary diesel fuel, because blends up to 5 percent fall within the standard diesel specification. Higher blends depend on the manufacturer. Many diesel pickups, vans and medium-duty trucks are approved for B20. Others, including some diesel passenger cars, have been limited to B5. Approvals can differ by model year, engine and market, so the same nameplate may carry different limits across generations.

The reliable way to confirm your limit is the owner's manual or the fuel filler label. The manual typically states the highest approved biodiesel blend. Using a higher blend than approved can create problems that the warranty may not cover.

Practical Considerations for Diesel Owners

Even in an approved diesel, biodiesel behaves a little differently from petroleum diesel. These differences grow larger as the blend percentage rises.

Cold weather: Biodiesel begins to cloud and gel at higher temperatures than petroleum diesel. In cold climates, suppliers often adjust blends seasonally or use additives, and higher blends need more care. Cleaning effect: Biodiesel acts as a mild solvent and can loosen old deposits in tanks and fuel lines. The first tanks after switching may clog the fuel filter, so an early filter change is a sensible precaution in an older vehicle. Seals and hoses: Some older diesel vehicles use rubber fuel-system parts that can degrade with high biodiesel blends. Newer vehicles generally use compatible materials. Water and storage: Biodiesel attracts moisture more readily and can degrade during long storage. This matters most for vehicles or equipment that sit unused for long periods. Fuel economy: Biodiesel contains slightly less energy per gallon than petroleum diesel. At low blends the difference is usually hard to notice, but it becomes more noticeable at high blends. Modern emissions systems: Diesels with particulate filters and related after-treatment were designed around specific fuel qualities, which is one reason manufacturers set blend limits.

What Happens If You Put the Wrong Fuel In

Putting biodiesel or diesel into a gasoline vehicle is less common, because diesel pump nozzles are usually larger than gasoline filler openings. When it does happen, the engine may run poorly, smoke heavily or stall. It may also foul spark plugs and the catalytic converter.

Putting gasoline into a diesel vehicle is the more serious mistake. Gasoline reduces lubrication in the high-pressure fuel pump and injectors. Modern common-rail diesel systems are especially vulnerable to this damage, and repairs can be extensive and expensive.

If you notice the mistake before starting the engine, do not start it or switch the ignition on, since that can activate the fuel pump. Have the vehicle towed or arrange a mobile fuel-drain service rather than driving it. Tell the technician exactly which fuel went in and roughly how much. If the engine was already started, shut it off as soon as it is safe and avoid restarting it. Ask whether the tank, lines and filters need flushing. Depending on how long the engine ran, the fuel pump and injectors may also need inspection.

Choosing by Situation

You own a gasoline car and want a lower-carbon fuel: biodiesel is not an option. Check whether your vehicle is a flex-fuel model that can use higher ethanol blends, or use the gasoline blend your manual specifies. You own a diesel and fill up at regular stations: you may already be using up to B5 with no special steps. You own a diesel and want to use B20 or more: confirm the approved maximum in your owner's manual, buy from a reputable supplier, and plan for cold-weather and filter considerations. You are buying a vehicle and biodiesel use matters to you: you will need a diesel. Check the specific model year's approved blend before purchase, because limits are not the same across brands or generations. You drive an older diesel: low blends are generally the safer choice. Consider having the fuel system and filters checked before moving to higher blends.

Bottom Line

Gasoline versus biodiesel is a question of engine type, not fuel preference. Gasoline belongs in spark-ignition engines, and biodiesel belongs in diesel engines within the blend limit the manufacturer sets. Know which engine you have, check the approved fuel in the owner's manual, and never mix the two fuel types across engines.