This is more of an extreme condition, but I'll give you my real world example.
On my Allison, I run a 28" 3-blade for top end, but I wanted something that would carry a load a little bit better since I usually have my family with me and I'm rarely in the boat by myself.
Anyway, I bought a 24" OT-X 4-blade from Hydromotive for this purpose and at around 5000 rpms, I was cruising at just shy of 60 mph, which wourked out to about 4-5% slip. My 3-blade 28" runs about 2-3 mph faster at the same RPM, but comes in around 10-12% slip. If my 3-blade prop was a 24" based on my results I would cruise at least 3 mph slower with the 3-blade at the same RPM.
Of course this is with my boat and these 2 particular props, but you can get the idea of how your fuel efficiency may go up or down depending on the prop you're running.
Your efficiency depends on a number of things and the only real way to know what the differences will be is to try different combinations.