what octane gas is better for smog test
And we certainly see a decrease in NOx without a significant increase in HC or CO. So, in some instances, buying a slightly higher octane fuel can both increase performance and help pass a failed emissions test. Generally, yes, spending money on higher octane than your owner's manual recommends is a waste of money. But sometimes ethanol tilts the scales. Of the 6 vehicles I've owned, 2 benefited from slightly higher octane-the other 4 suffered.