Yes, a car designed to run on E85 flex fuel can use regular 87 octane gasoline. The vehicle's engine control unit (ECU) is programmed to handle different fuel blends. It uses sensors, like the ethanol content sensor, to detect the fuel type and automatically adjust the engine's timing and fuel injection to prevent knocking and maintain performance. However, you will notice a significant drop in fuel economy when switching from E85 to 87 because gasoline contains more energy per gallon than ethanol.
The key technology that makes this possible is the flex-fuel sensor. This device measures the percentage of ethanol in the fuel line in real-time. The ECU then uses this data to optimize the air-fuel ratio and ignition timing. For example, E85 requires a much richer mixture (around 9.8:1 air-fuel ratio) compared to gasoline (14.7:1). The system is also designed to retard ignition timing if it detects lower-octane fuel to prevent engine knock, a damaging condition caused by premature fuel combustion.
While the car is engineered for this flexibility, it's not recommended to frequently mix fuels or switch between them without using up most of the tank first. Consistently using 87 in a flex-fuel vehicle that's predominantly run on E85 is fine, but you won't benefit from the higher octane (typically 100-105) of E85, which can provide more power in optimized engines. The primary trade-off is efficiency for convenience.
| Fuel Type | Typical Ethanol Content | Approx. Octane Rating | Impact on Fuel Economy (Compared to 87) |
|---|
| E85 | 51%-83% | 100-105 | Decrease of 15-30% |
| E15 | 15% | 88 | Minimal Decrease |
| Regular 87 | Up to 10% | 87 | Baseline |
| Unleaded 88 | 15% | 88 | Minimal Decrease |
| Premium 91/93 | Up to 10% | 91-93 | Slight Decrease (in non-turbo engines) |
