Difference between 3:55 and 3:23 gears

I have a 383 Dart GT (1967) and just put a 3.55 SG in it after removing the 2.45 open diff. I haven't replaced the speedo gear yet, so an indicated 90 equals about 63 MPH @ 3,000 RPM and 120 equals 83 MPH. That's with 245/60-14 Cooper Cobra tires. Thinking about taller tires 'cause the 3.55's and small diameter tires (25") have killed my highway gas mileage. 3.23's would prolly have been a better choice, but the 3.55's are great around town. What a difference! I'm thinking about some 15" or 16 inch wheels that would be about 28" tall. I have an 8.25 rear, so changing out the gears ain't as easy as an 8.75.