Been thinking of running 15" tires on my truck instead of 16".would that put a big dent in my mileage?

 

15" or 16" rims don't make a difference. You could run 22" rims if you wanted. The difference in mileage comes when deciding what rubber to wrap around those rims. A 32" tire will be better on mileage than say a 35" tire.
However, with 15" rims, you should see a massive gain in mpg's because you will never be able to brake because the rim won't fit over the caliper, and you'll have to remove the brakes to fit it on.

So it's a toss up really, brakes or 15" rims.

 

It has 16" on it now. I have 4 Good studed winter tires for it, that's why I was asking

 

15" rims (not tires - that would be in the Geo metro/suzuki swift size category) do not fit...Don't even know if you could find the correct bolt pattern in that size.

Sell your winter tires and buy a correctly sized set.