Octane isn’t power—it’s resistance to knocking. Knocking is the “ping” sound when fuel ignites too early. E10 usually has around 94 octane (Research Octane Number). Premium 91 is exactly that: 91. So mixing them raises the overall octane of the blend.
If your car’s manual says “use 91 or higher,” mixing is fine. You’re actually improving the fuel’s knock resistance. But if your car is designed for E10 only—like some flex-fuel vehicles—adding 91 won’t hurt, but it’s like feeding a vegetarian a burger. It works, but it’s not ideal.
Ask Away! with Jeff Smith: Some Calculations & Guidelines for Blending
Funny detail: older cars (pre-2000) might get confused. Their fuel systems can’t handle high ethanol blends well. Mixing 91 into E10 actually helps them by diluting the ethanol. It’s like giving an old man a little whiskey in his tea—calms things down.
What About Your Wallet?
E10 is cheaper than 91. Mixing them gives you a mid-grade fuel at a discount price. If you’re feeling thrifty, half a tank of E10 topped off with 91 saves money. Your engine won’t judge you. It just burns it.
But here’s a twist: ethanol has less energy per gallon than pure gasoline. So E10 gives you slightly worse fuel economy. Mixing with 91 boosts the energy density. You might get a few extra miles per gallon. Not huge, but enough for a smug smile at the pump.
One more thing: ethanol is corrosive to rubber and aluminum. If your car has old seals, E10 can cause leaks. Adding 91 reduces the ethanol percentage, which is like putting a band-aid on an old wound. It helps, but doesn’t cure it.
