Remember, plugs are due every 30k miles OR 3 years, whichever comes first. Stop & Go and idling of city driving takes its toll.... 17-20k city miles are just as bad, if not worse than 30k of highway mileage.
I think you can make a case that 20k miles of city driving is more like 60k miles of highway. Because its the cold starts and warm-up that are toughest on the motor, and oil, people typical travel 4 or 5 times as far between start-ups on highway miles than city miles, and the time spent, stop and go, braking, accellerating and sitting idling the motor count as well.
Originally Posted by jeep5253
If you are using premium gasoline, don't. The Owner's Manual says not to use it. 89 octane gas is recommended, with 87 octane being acceptable.
Yes, higher than needed octane gasoline gets you absolutely nothing, except wasting money. Unless there is something wrong with your vehicle or you modified the motor extremely, or live an extriodinarily hotter and denser air environment than the designers ever envisioned (like Death Valley or Kuwait in Summer), the recommend Octane is more than enough.
People have shown lower octane gets better mileage, my guess, the lower volatility allows better atomization and more efficient burning.
The only way lower Octane is bad, is if you get spark knock, actual detonation, which is really bad for the motor and can damage it, that should NOT happen if you're using recommended octane. The Engine Controller has a safety feature to recognize the spark knock and/or detonation, and retarnd ignition timing to stop it. BUT, retarding ignition timing kills power and power for the amount of fuel burned, resulting in mileage dropping.
So, if your Owner's manual says to use 89 Octane, with 87 Octane being acceptable. That tells me, 89 is best, but the motor will NOT suffer damage if you use 87 octane. I'd measure mileage for both octanes, I would NOT be surprised if the controller has to retard the ignition often for 87 octane, resulting in worse mileage, if it does NOT, which it wouldn't for colder temperatures or higher altitudes, then 87 octane might provide better mileage. Eitherway, measure it and go with the Octane that produces the best for the conditions, don't use octanes outside of what is recommended or acceptable, it can damage the engine. Too high octane, NOT very likely to do any damage, but if it changes anything it will make mileage and cold starting worse, so your just wasting money for worse performance with higher than recomended octane.