“E85 makes rubber swell the most”
Backwards. Elastomer swelling PEAKS at low ethanol — around CE17a. At CE85 volume swell returned to baseline or below. The real failure mode is plasticiser extraction, which embrittles a line with no swelling at all: it can pass a squeeze test and still be near failure.
“Ethanol corrodes all fuel-system metals”
Uniform corrosion was under 10 µm/year for nearly every metal tested in E85. Damage concentrated at galvanic couples — zinc or lead against steel, nickel against aluminium. Ethanol’s role is raising conductivity, which makes an existing couple work harder.
“You’ll lose 15% fuel economy”
EPA’s own data across 1,538 factory flex-fuel vehicles puts the median at 26.3%. Vendors quote the low end. We quote the measured one, because it’s the number that decides whether converting pays.
“E85 is 105 octane”
The Renewable Fuels Association — the ethanol industry’s own trade body — says this is wrong, and explains why: 105 comes from applying ethanol’s blending octane value, which only describes what the first few percent of ethanol do to gasoline. Engine tests put E85 at 94–96 AKI. That is still above 93 premium, so the honest number tells the same story. We had this wrong on this page until we checked.
“A conversion kit adds 5–20% power, no tuning needed”
Not on most cars. With timing and boost unchanged, the fuel swap alone is worth about 1% — gasoline and E85 carry nearly identical energy per unit of air. Octane only pays back where the ECU was already retarding for knock, which means boost. Oak Ridge measured 5% on a stock turbo GDI car and nothing at all on one of the port-injected ones.
“A converted car starts in the cold just like a factory flex-fuel one”
A factory FFV ships with a high-ethanol cold-start strategy in its calibration. A converted car does not — the kit widens injector pulse from a sensor reading, it doesn’t author a cold-start map. Ethanol’s low vapour pressure and high heat of vaporisation are exactly what make it reluctant to light in a cold cylinder, which is why ASTM D5798 defines four seasonal volatility classes and winter “E85” is cut toward E51. Expect longer cranking on a hard frost, and keep some gasoline in the tank through a cold snap.
Above 93 premium, and genuinely useful. Not the '105' everyone repeats — the ethanol industry's own association says that figure misapplies ethanol's blending octane, and engine tests put E85 at 94–96 (R+M)/2.
Against 14.7:1 for gasoline. The engine needs materially more fuel per unit air — everything else follows from this.
Peak fuel-system flow increase. Hardware is sized to this. It is not the economy number.
Median across 1,538 factory FFVs in EPA's certification data. Vendors quote 15–25%; the measured figure is worse, and we plan against the measured one.
'E85' is a seasonal spec, not a constant. Winter blends cut ethanol to help cold starts — which is exactly why a sensor beats a fixed tune.
E85 must be at least this much cheaper per gallon than the gasoline you buy today, or converting costs you money. At April 2026 national averages the real spread was 19%.
Now check your own car.
Thirty seconds, no account, and it will tell you if the answer is no.