I have 16" tires and rims on my 1500 dodge ram 4x4 can i put 15"rims and tires on it
Well, yeah, you can if you really want to, it's entirely possible but avoid adapters. The wrong kind will throw off the load carrying spot if the adapters move the rim out, and destroy your wheel bearings. Get 15's with the same lug pattern and if possible get a higher profile tire so it's as close to stock as far as # of wheel turns vs. distance traveled
Make sure the rims will fit over the front calipers before mounting tires on them. Use this site for determining the correct tire size - http://www.miata.net/garage/tirecalc.html
if it's a newer model no. the rims won't clear the calipers.