Tires: changing tire size to change gear ratio, chevy 3500, ring and pinions

Question
I have done research to find the effects of a size change, but want and opinion before I make the wrong move.  My vehicle: 1998 Chevy 3500 crew 4X4 dually 7.4L automatic 4.10 gear 225/70/16.  At 70 mph I turn 2500 rpm with plenty of power.  I want to change to 235/85/16 to try and get a higher ratio of final drive.  I calculate that the new tire will be 2.5" taller. That equates to a 1.1% reduction to 2270 RPM.  Will this help economy or will the taller defeat savings.  Like I said, there is plenty of availabe power and I feel that the vehicle is "overworking" for the speed.  I don't tow that often, and this is cheaper than changing ring and pinions.  I am aware of possible clearance issues between dual rear tires and speedo difference.  

Answer
Jon,

I hope you meant the tire size was LT225/75R16 - not 70 - as this was the original tire size on the vehicle (if my book is correct).  If you happen to have P225/70R16's on this truck remove them immediately - they don't have enough load capacity and are likely to fail.  

Assuming you have LT225/75R16's, it seems logical that you would get better fuel economy, but with a reduction in pulling power - but this is more of an engine issue and I can't speak for how your particular engine will behave.

And the dual spacing issue is of concern.  If the duallies touch, it causes a very strange failure mode that is almost impossible to diagnose unless you've seen it before.  So make sure you get that part right.