Thanks to various state incentives here in the heartland, ninety one octane E-10 premium gas is up to ten or twelve cents per gallon cheaper than eighty nine octane regular fuel. I know that Click and Clack are pretty adamant that very few cars actually need higher octane fuel but given the price difference, is there any reason for me to not use whichever is cheaper?
Here in New Jersey gas is 10% ethanol, regardless of octane, since MTBE was phased out. The only effect I’ve noticed is a drop in mpg (~1 to 3 mpg). I don’t think it would hurt to use it but the drop in mileage would probably negate any savings. Check your owners manual, it should tell you how much ethanol the car can handle.
There are two down sides. E-10 (10% ethanol) has less energy per gallon than gasoline so there is a slight loss of mileage. Not likely 12? worth. There is also a chance that it could cause some damage to your car. Using premium in a few cars that are designed for regular can cause damage, but it is not common and 91 octane is not that high anyway. I would suggest that you will lost maybe 2% of your mileage so as long as it is more than 2% cheaper, I would go for it.
EVERY time I’ve used E-10 in my car I’ve suffered a 10% mileage decrease. You’d have to do the math to figure out if the cheaper price actually saves you any money. All I can say is I dread the day all gas pumps have ethanol blends in them.
As far as octane goes, using fuel with more than you engine requires is not harmful, but it won’t help anything, either. Higher octane gasoline is not “better” than lower octane gas, it just has more octane.
In my area regular gas is 87 octane. 89 is considered “mid-grade,” and premium is 91 octane.
Try a couple of tanks and see what your MPG is. Compare how much it costs to go a tankful with E10 91 octane and the MTBE 89 octane. Use whichever is cheapest.