I have an 04 Lariat 5.4 (2WD) with the 3.73 gears. Due to a whining rear end, I am having the ring and pinion replaced under warranty and my buddy is the service manager at the local Ford dealer so he said I could put whatever I want in there. I am thinking about changing from the 3.73 to the 3.55 because I rarely tow anything, and would think I could pick up a few more MPG's and perhaps a quieter highway ride (thanks to lower rpms). What do you guys think??

 

I have the 3.55, and I'd swap you in a minute. The extra power is well worth the fraction of a mile to a gallon you would actually save. The around town mileage is often lower with the 3.55. Also, if you wanted to gear it up you could put bigger tires on it. To gear it down you have to put too small tires on. So the 3.73 gives you more flexability. The 3.55 does not.
Chris

 

I tow my 16 foot boat occasionally with the 3.55 LS and have plenty of power. The only down side to 3.55s though is if you want to go up in tire size you may notice more of a power loss than if you would with the 3.73. Overall, I think there really pretty close. Good luck.

 

stay with 3.73. These trucks are tanks.

 

I'd only go with the 3.73's if I couldn't talk them into installing 4.11's.