Mustangs, from the very first day, have been about customization -- making a car feel unique to one driver, like no other on the road. Performance is as much a part of that equation as anything else, and the particular balance of one type of performance over another can be as subjective as anything else. That goes for bolting on tires, building motors, and even swapping gears.
Which gearset is right for you depends on a lot of things, not least being the car's weight, its engine, transmission and anything else you've done to it. There's only about a 5 percent difference in ratio between these two gearsets, so don't expect a set of 3.73 gears to turn a slow-coach into a track beast. At best, the taller gears can help the car to feel a bit more perky off the line, and more lively around town; the gearing upgrade is also good for owners who have upgraded to big rear tires, and who don't want to sacrifice their 60-foot times for a gain in traction. A swap up to 3.73s will give you back the wheel torque you'd lose if you were to step up from daily-driver 45-series tires to more track-oriented 55-series rubber.
With the Mustang's overdrive top gear -- present since the earliest Fox bodies -- the difference in cruise rpm and fuel economy at cruise should be negligible. You're looking at going up by about 70 to 100 rpm at cruise, which isn't enough to make a marked difference in economy. Newer, modular-engine Mustangs -- which are higher-revving and heavier -- can probably stand to benefit more from a 3.73 swap than older Mustangs, with their lower weight and push-rod-engine torque. That said, most Fox body owners who have stepped up from 3.55 to 3.73 gears haven't regretted doing it.
