Fresh plugs will improve mileage that has degraded gradually over the 10's of thousands of miles. Platinum itself will NOT improve the mileage. Of course there are hundreds of different engines and plugs, so its always possible to find one combination that works a little better than others, but usually the general rules hold true.
Like you stated, platinum lasts longer longer than the standard steel electrode plugs.
Problem is, I'm pretty sure all the Commander engines use a wasted spark ignition system, where 2 plugs share a coil. This kind of system has a reverse current flow for half the plugs, and the reverse current flow will reverse the anode and cathode of the plug. Normally the current flows from the Platinum to the Steel, and less metal is carried with the current. Reverse the current flow and it flows from the steel to the platinum and in that case a lot more metal is carried from the steel to the platinum. In short, with a platinum plug, reversing the current eats the steal portion of the steel away much faster than just plain steel plugs.
Double Platinum plugs have platinum on both electrodes, so when you reverse the current flow it still flows platinum to platinum, and NOT from steel to platinum, where it carries a lot of steel with it.
And yes, keeping tire pressure up does a lot to improve mileage, as well as a clean air filter.
Alcohol has about 1/2 the energy density that gasoline does, which means you have to burn twice as much to make the same amount of power. The exact ratio varies with the type of alcohol.
Keep in mind, ethanol in high enough ratio can be corrosive to seals, metals and hoses in the fuel system. So do NOT use E85 unless you confirmed your fuel system is designed to use it. ON top of that, check all the manufacturers recommendations about using E85, other Chrysler Flex Fuel Vehicles, they recommended using a special motor oil with the fuel. I've seen lots of warnings about the alcohol causes wash out of oil on the cylinder walls, blow-by contaminating oil, etc.
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Last edited by Mongo; 02-09-2011 at 10:18 PM.