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Engines that are designed for high octane gas (turbo Saabs and others) perform best with that gas. When you use lower octane, the engine will knock. The ECU will then back off the performance to stop the knocking. Power drops off concurrently. You step on the gas more to get the same power and therefore use more gas at lower octane.
At least that's the theory. In real life, it depends on how you drive and where. Some drivers will actually save money with premium, some will save with lower octane (I would not use 87, but 89 might be OK).
Let's say Premium is selling for $2.50. Premium usually costs .10 more per gallon than 89 octane. That's a discount of 4% off the price for 89 octane. So, if your mileage drops more that 4% with 89 octane, it's a net loss for you.
As a practical example, let's assume you normally get 30mpg. If you put in 89 octane and get less than 28.8mpg (4% less), you lose money. As you can see, It doesn't take much of an mpg loss to lose money on regular.
As the price of gas rises, it gets much less attractive to use 89 octane since the price differential between 89 and premium always seems to be .10 cents. For example, at $4/gal for premium, using 89 octane is a savings of 2.5%. So, if your 30mpg car drops to 29.25mpg or less, you lose money.
Or, how about we annualize it. Let's say you get 30mpg with $3.00 premium and drive 15K miles per year. You'll use 500 gallons of gas and pay $1500 for gas over the year. Now, let's assume 89 octane is .10 less but you lose 1 mpg (just 1 mpg). You'll use 517 gallons of gas and you'll pay $1499.30 for gas. Whoops - you don't save the cost of a cup of coffee.
Or let's go extreme. Premimum at $2.50. 89 Octane for $2.40 . You get 30 mpg with premium, 29.5mpg with 89 octane. You drive 15K miles/year. Your total gas used with Premium is again 500 gallons at $2.50 for a total cost of $1250. Your total cost with 89 octane is $1220. Total savings $30 per year. Whoopie. Even if we drop to 87 octane at $2.30, and even if we still predict that you only lose .5 mpg as above, you pay $1168 - a savings of only $82 (and that's the extreme example with low gas prices and almost no MPG loss).
So, you just drove your fabulous turbocharged Saab in a "reduced power" configuration and missed out all the turbo fun for an entire year to save $30 - or maybe in 87 octane slug format to save $82. Bottom line, it ain't worth it.
posted by 98.110.150...
, Sun, 16 Nov 2008 09:30:38
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