That's what I would do.I am willing to bet that unless you are towing, off roading or rock climbing 87 is fine.
If I tow then I'll use 89 to be safe.
Octane should not affect MPG in the Cherokee's case.
The only way 89 is going to improve performance is if 87 is causing pinging, in which case the detonation sensor is going to signal the ECU to retard timing, decreasing engine output and performance. Outside of that, no one will see ANY difference.
Now, if the price were negligible, I would use BP 89, but only because it isn't mixed with ethanol at my local station. But the price difference isn't negligible.
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