Jeep Compass octane

That should read 89.

Assuming USA Octane ratings (which are the average of research octane and motor octane values), it won't make a difference in almost all situations.

The octane rating is merely the tendency to ping (or not ping) which happens when the compression ratio or heat or load is such that the flame front proceeds too quickly.

The spark timing can be retarded in almost all (if not all) modern engines to prevent any damage from occurring (due to knock sensor input & electronic ignition system output).

The worst that will happen in most modern engines is slightly retarded timing under maximum load (e.g., accelerating up a hill or towing a load).

What will really happen in almost all circumstances is... nothing.

You might see a slight decrease in gas mileage and power due to the slightly retarded spark timing . Modern electronic controls will optimize it all .

Years ago I ran a test on a vehicle which stated it needed premium and I couldn't "measure" any difference whatsoever in gas mileage, even as I agree fully with your premise that there "should" be a "slight decrease" in gas mileage.

I think the reason I couldn't measure it was that the measurement system we typically use isn't precise enough because the decrease is too slight. We measure by the typical method of what I refer to as "gallons used" tests. a. Fill the tank b. Drive normally until you need to fill the tank again (note the miles) c. Fill the tank again (note the number of gallons used & divide miles/gal)

In my experience, this "gallons used" method isn't precise enough to measure the "slight decrease" you'd expect from lower octane rated fuel.

When they really test gas mileage, they put a known WEIGHT of fuel in a plastic bag attached to the fuel lines, and run that and then weigh the fuel after a set number of miles. Our gallons-used test is less precise.

Also, we don't normalize the type of driving, where in general, it's my understanding, that the only time the "wrong" gas should knock is in when the spark is advanced which doesn't happen all that often in normal driving.

Sure, if you race everyone out of a stoplight, you'll be advanced, but most normal driving doesn't use spark advance all that much (although all are advanced - but I'm referring to extra degrees of spark advance situations).

This method is accurate - if you go like 10 tanks the differences average out . One tank at a time will give you a "close enough" mileage number .

Your understanding is outdated . Modern controls will optimize spark advance and fuel mixture for the best possible combustion . Now my '86 GMC pickup with old fashioned distributor CAN be over-advanced and made to ping . A note about that - the original distributor did have a mechanism to retard spark if a knock was detected . But the block , heads , and connecting rods are about the only stock parts left .