Does anyone know how to get an idea of what amount of speed increase i will get if i go from a 75hp to a 115hp. looking at new boat but i like the one i have just want to move a little faster. Thanks

Too many variables.  What kind of boat?  Prop pitch, # of blades etc...? 

Gonna depend on the boat.  If you have 75 on a 17' boat and put a 115 on same make or similar style 18' boat, you may only see very little, if any difference.

You should stay with the max recommended HP if you want good performance.  No more than a 10% reduction from max, or you will start big drops in hole shot and all around performance.   

If you go from a 75 to a 115 on your boat and it doesnt put it over the  HP Rating your loking at 5-8 mph

On my 1980 Skeeter SS-1 15' 6" boat it would do 50 MPH with the 115 HP Evinrude.  With a 70 HP Evinrude the max speed was 36 MPH.  That was a small fiberglass boat.  With an aluminum boat you will probably see a simular result.  My partners 17' hydra-sport did 54 MPH with an Evinrude 115HP.  I would expect high 40s to low 50s. 

Thanks to the replies. My max speed is 34 on gps w/75 hp was thinking of going up to 115hp.