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No Surprise: E85 Is a Bummer in Fuel Economy

We did a comparison test of two fuels, regular gasoline (87 octane) and E85 (100 to 105 octane). Our test vehicle was a flex-fuel 2007 Chevrolet Tahoe 4WD LT powered by a 5.3-liter V-8 hooked to a four-speed automatic transmission.

We tested acceleration using both fuels and our standard procedures, then we measured fuel economy at steady speeds of 30, 50, and 70 mph around a 2.5-mile oval test track, three runs at each speed that were averaged to produce the numbers you see in the accompanying charts. The fuel-economy results were calculated using the vehicle’s onboard computer.

We began the test with the Tahoe running on E85 fuel and later drove the SUV until its tank was as empty as we dared, and in that way we were able to flush the tank of almost all the ethanol. Then we refilled the tank with regular gasoline and repeated our procedures. All testing was done in two-wheel-drive mode. The results are shown here.

Differences in acceleration times were insignificant (although GM says E85 improves horsepower by as much as three percent). On the downside, the fuel economy on E85 was diminished more than 30 percent in two of the three tests, about what we expected. The EPA’s numbers suggest that fuel economy worsens by 28 percent on E85 compared with regular gas. On any Tahoe equipped with a 5.3-liter V-8, the E85 flex-fuel feature is a no-cost option, but running E85 reduces the driving range from roughly 390 miles a tank to about 290.