Were any non-cosmetic changes made to the Chrysler 300M between 1999 and 2004? My 1999 with 210,000 miles gets better mileage (about 10% to 15%)and has a bit more power than my 2004 which has only 55,000 miles on it. I had thought that they were nearly identical mechanically. In fact, that is why I bought the newer one. The old one is such a marvelous car: great mileage, plenty of power and speed, superior traction, luxurious appointments.
If anything the newer one should be slightly faster provided it is a 300M “Special”. Apparently the “special” had slightly different ECU tuning, (to take advantage of premium fuel), dual exhaust, and a shorter 3.89 axle rato. If you do have a “Special” and are just using 87 octane it then that would explain the lower power and fuel economy. It the 2004 is the standard version, then it should perform the same as the older one.
If you think the front wheel drive 300M’s were great cars, take one of the newer rear-wheel drive, Hemi-powered models out for a spin. Your front-driver will feel like a Neon by comparison