not only will the online calculators give you speedometer differences, there are a bunch of free smartphone speedometer apps that can give you real time speed info...
for my 2004 Foz with stock size all seasons - the speedo reads about 3mph faster than actual... meaning when the needle is on 30mph i am actually only going about 27, give or take.. verified by a local municipality and their portable radar speed sign... you know the ones.. gives posted speed limit on the upper half with a radar reading of your speed on the lower half.. so a slightly taller tire might actually make the speedo more accurate, but yeah, it does depend on the actual tire size...
unless you are getting really crazy with size changes, weight is really not going to be that big of a factor - 1 or 2 lbs/per is negligible in most situations - basically the equivalent of a few bags of groceries or a few extra tools - big deal.
Tire compound will make more of an impact.. how hard or soft... harder compound (longer wearing - like touring) will give better fuel mileage, but wont be as grippy on slippery roads... softer compounds (like snow tires) will grip better, but wont last as long (also reduces mpgs) - finding the right balance of grip and durability for your area is the real challenge.
Up north here, where we have winter conditions for more than half the year, it makes sense to have dedicated snow tires, and many of us do... down south, not so much. All depends on where you live and travel.
that is my 0.05 cents worth (increased from 0.02 for inflation)