The tire diameter is what matters. It will change the revolutions per mile, and change the RPM at a given speed.
Low profile tires are not generally good for mileage nor are they cheap to buy. Keep that in mind as well. Stay with as narrow a tire as is recommended for the new rim.
Plug your info in here to see what will happen. this is one of many calculators on the web.
http://www.roadkillcustoms.com/hot-r...#axzz2cp6K0hgz
__________________
2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle
currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears