There's nothing "wrong" with 3.73s as a bolt on for 5.0s, but they don't provide a "performance improvement" in specific applications over a better gear choice when 3.73 gears don't meet those needs, say like a stock 5.0 with 3.08s in a drag race, which will run faster running through the traps in 3rd gear (which it is built to do) than in 4th with 3.73s.
The reason I poked fun at 3.73s is because they are a popular performance mod that people put in their cars without understanding that it doesn't work as well as they might think for all or even most applications. You might notice a "seat of the pants" improvement on the street when compared to stock or other higher (numerically lower) gearing when using the same transmission gear, but it doesn't actually improve performance for say, straights or corners on a road course or quarter mile time...especially when that gearing for, as an example, a completely stock 5.0 Mustang will only provide excessive wheel spin under hard launch (or hard throttle coming out of a corner) where there was none before.
Realistically, if a driver wants more seat of the pants feel, instead of swapping gears, all that would be needed is to downshift, run the transmission gear out further or launch at a higher RPM and use clutch modulation to keep wheel spin from occuring.
Ultimately, for specific performance applications a mathematically accurate gearing choice is much better than a random gear choice. Ford did this with the 5.0 from the factory, as I stated above, by gearing the car to finish a drag in third gear while still providing traction with the smaller/less sticky (than serious performance tires) tires.
In the case of the stock 5.0 Mustang, where the stock tire HEIGHT is used, but with more width (or a transmission with a numerically lower 1st gear such as a T5Z), you would see better acceleration over the entire quarter mile with 4.10 gears (provided the extra shift wasn't something that slowed the car down because the driver couldn't perform it quickly and smoothly enough), which is the gear that will allow the stock motor to provide the most power over the distance of a quarter mile drag (the stock 3.08/3rd gear combination is 3.94 for 85-89 cars and 4.12 for 90-95 cars). 3.73s or 3.55s don't allow the motor to fully spin in 4th gear in a stock 5.0 configuration and thus the motor doesn't get a chance to provide the most propulsion at the end of the track like the 3.08s in third...and they add an extra shift (which is time the car doesn't see power going down the track). But the key here is in a STOCK APPLICATION for DRAG RACING. Once the drivetrain is modified or the performance application changes, the gearing need is probably going to change...and in surprising ways.
Ryan
