going from a 2.90 to a 3.91 is going to be noticed. depending on tire size.

my 88-440 truck turns a 29x15x15 M/T 2600 RPM @ 55 MPH with 3.91sg gets 10 MPG

2.76 turns 2000 RPM @ 55 MPH gets 12.5 MPG but topend is UNLIMITED! 4800 RPM @ 130 MPH flying mile.