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Old 01-29-2016, 07:59 AM
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Join Date: Dec 2012
Posts: 36
E5 87 octane -vs 87 Octane -vs- 91 Octane Experiment

I decided to run a little test using different fuels in my 1974 280C. Find following the results:
E5 (5% Ethanol) 87 Octane 15.2 mpg $.112/mile
87 Octane (no ethanol) 16.2 mpg $.113/mile
91 Octane (no ethanol) 18.7 mpg $.108/mile

The test is probably not totally accurate. I only ran 1 tankful of the E5 and 91 octane and 2 tanks worth of the 87 octane. So there could be a lot of variability. So, I will keep track of it over the next few months. I live in Nebraska and the last week (91 Octane) has been considerably warmer (average temperature in the 30's) as compared to the earlier tests (average temperature in the 20's) so this might affect it some also. The costs for the fuel were recorded on the same day so fluctuation in prices is eliminated for the $/mile. The car does seem to run and start better with the 91 octane, but this could also be attributed some to the change in temperatures (fuel injection is a wonderful thing).
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Old 01-29-2016, 12:43 PM
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Join Date: Dec 2007
Location: Houston, Texas
Posts: 1,148
Hi Grubby,

Glad to see comparison showing Premium fuel is actually less expensive by actual mileage outcome. There could also be less obvious costs to using lower grade fuel if mechanical components suffer and wear or become damaged.

It is possible to change the timing on carburetor engines to run on regular unleaded without knocking or pinging. Premium fuel is recommended by Mercedes as they designed the engines for higher octane.

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