Low vacuum question

There's the other one I was hoping to here something from. The reason I figured the 3.55's was my Dakota has them and in Drive (not OD) it turns 2500 plus at 60 mph with 28" tires. I'm plan on driving this Cuda quite a bit on cruises and am hoping to get at least a little bit of fuel economy from it. If I keep the 4.10's it has now it's turning 3100 plus at 60 mph and my guess is fuel economy is gonna suck. Pardon the Pun. 3.90's wouldn't do much for lowering the rpm's so I didn't think I had much other option. I haven't bought the new back tires yet and was tossing around 27's or 28's. Haven't figured up the RPM diff. between them yet. What are your thought's Rumble?