Ethanol almost never reaches a driver on its own; it arrives blended into gasoline. The blend is labelled with an “E” and a number — the percentage of ethanol — and that small number carries a surprising amount of engineering and policy.
The blends
E10 — up to 10% ethanol — is the default fuel across most of the world and is approved for essentially every modern petrol engine. E15 (15%) is approved in the US for model-year 2001 and newer light-duty vehicles; it offers a route to more ethanol per gallon but has faced recurring rules over summertime sale. E85 is a high-level blend of 51–83% ethanol (the figure shifts with season and region) and is strictly for flex-fuel vehicles — cars whose fuel systems and engine software are built to handle any ethanol proportion from zero to E85.
Which vehicles can use what
Low blends are universal; high blends are not. Any modern petrol car runs on E10, and US-approved vehicles from 2001 on can take E15. But E85 requires a flex-fuel vehicle: ordinary engines are not built for the materials compatibility and fuelling calibration that high ethanol concentrations demand. This vehicle-capability gap is precisely what fuel-choice proposals such as the Open Fuel Standard sought to close.
Fuel economy and handling
Ethanol holds less energy per gallon than gasoline, so fuel economy slips a little as the blend rises — negligibly at E10, noticeably at E85 (often offset by lower pump prices). Ethanol also attracts water and is more aggressive toward some seals and metals, which is why blend limits and material specifications exist in the first place.
Biodiesel blends: the B-numbers
Diesel fuel uses the same shorthand with a “B”. B5 (5% biodiesel) is widely sold as ordinary diesel in many markets and is approved by most engine makers; B20 (20%) is common in fleets and generally accepted with minor caveats; and B100 (pure biodiesel) is used only in compatible engines with careful cold-weather management. As with ethanol, the constraints are material compatibility and cold-flow behaviour, and the same idea applies: low blends drop in unnoticed, high blends need the right equipment. Renewable diesel, by contrast, is a drop-in fuel and can be used at far higher levels without these blend limits.
Why blend levels are capped
Blend ceilings are not arbitrary. They reflect three real constraints. Materials: ethanol and FAME can degrade seals, gaskets and certain metals not designed for them, so engines must be built or certified for higher blends. Cold weather and water: ethanol attracts moisture and biodiesel gels in the cold, both of which worsen with concentration. And warranty and certification: vehicle manufacturers approve specific maximum blends, and regulators approve fuels for specific vehicle populations — which is why E15’s approval is tied to model-year 2001-and-newer vehicles. The blend label at the pump is, in effect, a compatibility promise as much as a recipe.
The blend wall and policy
Because nearly all fuel is E10 and the fleet is built around it, there is a practical ceiling — the blend wall — on how much ethanol the market can absorb. The Renewable Fuel Standard pushes volumes up against that wall, which is why the availability of E15 and E85, and the number of flex-fuel vehicles on the road, matter so much: they are the release valves that let ethanol use grow beyond 10% of the gasoline pool. Higher blends also depend on retail infrastructure — pumps, tanks and dispensers rated for the fuel — which spreads slowly because it requires investment by fuel retailers, not just by producers. The interplay of mandate, vehicle capability and pump availability is what ultimately sets how much ethanol reaches drivers.