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85 Coupe Deville 4.1 V8 & 4.3 V6 Deisel & 1970 Eldorado
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776 Posts
Wile i agree with the statement "There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs", There can be a benefit to not using a fuel with an Octane rating that is too low. Many of todays higher performance engines may have there octane requirements increase over time due to carbon buildup raising compression slightly, Hot spots developing within the combustion chamber, Worn plugs,etc. This can cause pre -ignition. Many of todays engine management systems will detect this and retard the timing to compensate. This can decrease fuel mileage. When a higher octane is used, Higher mileage can be seen. This is not because premium has any more energy but that the engine is running closer peak efficiencies.

41 MPG Highway (The best trip tested)
30 City (Town really)
85 HP 1985 Coupe Deville V6 Diesel
Just don't try to pass! But it runs on B100
 
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