Octane and Altitude

87 octane is mid grade here whereas 85 is regular. When we first moved to Colorado, the vehicle we had took some time getting used to 85 as it had been run on 87 for the first 4 years of its life while at sea level. Our new vehicle runs just fine on 85 as it hasn't had any higher octane fuel for the ECM to calibrate to.

carringb wrote:
If I'm towing, I make sure I fill up with 87 before heading back downhill.

You don't need 87 octane going down hill. You barely need gas. :)

What vehicle/motor are we talking about? Most if not all modern engines have knock sensors that will retard timing if knocking occurs.

I am on my 2nd Chevy Silverado, 5.3 L. V-8 and neither manual says a thing about lowering octane for altitude. I've tried it both ways and once my "service engine" light came on with the 85 octane. My theory is that the idea of lower octane is outdated with the newer motors that adjust to various conditions so one should stick with the 87. But I'm no engineer. Problem is that 87 is no longer the lowest price in those places.

If I'm towing, I make sure I fill up with 87 before heading back downhill.

You will need the required octane for the elevation where you are operating the vehicle. If you will still have the low octane in the tank at lower elevations, then you should fuel up with higher octane first.

The engine may not ping like the old cars did but the knock sensor will retard the timing which will increase fuel use and reduce power somewhat.