The fwd fuse is not intended as a full-time conversion to fwd (only for 2-wheel towing) and only works on the autos. I believe leaving it in can burn out a duty solenoid or something, but I don't remember the discussion I half-followed a few years ago. The best way to convert is to swap in a fwd transmission from an earlier Legacy or Impreza. They're all the same as far as installation goes (Subarus are like Legos). The first-generation Legacy manual transmission also has the benefit of being 3.70 FD, the tallest of anything introduced since the Legacy (with the possible exception of the SVX, which has a rear FD of 3.55, but it may have a 1.1 center differential like the later Imprezas and, IIRC, a 3.90 FD for the front differential. I'm too lazy to confirm with the math, but that is the closest ratio, so I'd bet that's it. This is all irrelevant anyway). Don't know about the autos (I think they're 3.90 FD, maybe 4.11, don't know gear ratios, but I hate autos so I don't follow them). A used fwd transmission should run you no more than a couple hundred dollars and is an easy day swap (theoretically, I make no claim to the ease of removing stuff that's been in place for about 15 years). From there, you can either swap out the rear suspension for the 2wd version's to save more weight (struts and rear hubs, the rest can remain), or leave the rear differential and axles in place. Keep in mind that the former option leaves less options for suspension changes (the strut mounting to the hub carrier is different) if you're into that. KYB GR2s would be about the most aggressive thing you can get for the rear, but they're quite good for general road use.
Mileage gain? Not being able to compare as I've yet to convert to AWD (and no longer sure I want to, not that it would matter anyway as it'll be turbocharged by then), and finding people report wide variations in mileage, I'd guess 2-3 mpg. FWIW, driving like I stole it, I averaged 25-27 mpg a tank on my fwd 1990 Legacy wagon.
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