This is not totally true... for highway driving yes but anywhere that you can drive 60 mph or less they are good because you can get into 6th gear sooner. Where I live I average 21 mpg with 3.73s.
Guess it depends on the speed limit. Where I live the limit is mostly 60 km/h (37 mph). When I am in fifth at that speed I am over 2000 RPM. However, when I shift into 6th I lug the engine. So I spend most of my time in 5th at ~2000 RPM. Not the most efficient way to drive but there is no other way with the 3.73s. 70 km/h seems like the sweet spot in terms of fuel economy for this car with the 3.73s.
2018 GT350 - Race red
2012 Mustang GT/CS - Race red - Electronics package - Security package - 3.73 - Steeda shift knob - Steeda shift bracket - ProCal II Tune - GT500 ABs - Shaftmasters DS - Boss 302S Wheels - Build date: February 14th, 2012
2011 Mustang V6 - Pony Package Premium Coupe - Black - Charcoal Leather - Roush Front Fascia - Matte Black Lemans and Hockey Stick Hood Stripes - 6% HP Charcoal Tint - 3.31