Just wanted to know if anyone has used these, or any other underdrive pulley on their magnum. Interested in getting them but not sure if it's worth the money?

 

I don't think you're going to see any noticeable improvement in MPG with underdrive pulleys. Underdrives are really meant for highly modified engines where you're looking for that extra 1/100 of a second in the 1/4 mile. On the street you can actually run into problems if you underdrive your alternator and/or AC compressor. The alternator may not be able to keep up with the electrical demand of the car if you are cruising at low speeds/RPM's, and the same with the AC compressor. Never underdrive a power steering pump. The only time I've ever used underdrive pulleys was on a 1980 Chevy Monza with a 240 hp 4-banger, and even then I didn't notice any difference in acceleration times or mpg. Underdrives might be practical if being used on a tow vehicle that spends most of its time at highway speeds/RPM's.