want to put smaller wheels on dakota r/t?

From the tire aspect, the key to changing wheel diameter is to pick a tire that will give you the same overall height (primarily) and width (secondary). This can easily be done by playing section width and aspect ratio. HOWEVER, going to a smaller rim size can cause caliper interference. I had a 98 Dakota with 15s and when I had new tires balanced with stick on weights on the inside of the rim, they rubbed on the caliper. Since your radius will be decreasing by 1/2 inch, You will need a clearance greater than that with your current setup. Get under the car and measure. if you have 9/16 inch clearance now, that will become 1/16 when going to a smaller wheel. There are other things to watch out for too; primarily steering linkage clearance. Do you have access to a 16" Dakota rim? Try going to a salvage yard and seeing if they have a 16" rim with a tire on it. If you talk nice, they just might let you take it out into the parking lot and test fit it. We have a GREAT salvage yard here called AA that will gladly let you do that.