I just installed a Suzuki 140 on my 1974 19-6 mounted on a Bob's setback bracket with a Bob's Jack plate mounted to the setback bracket. Same setup that held my Johnson 150 for 6 years.
I started with a full tank. Filled up my 47-48 gallon tank, full cooler, my wife and I in the boat. Here's what I have so far:
23 pitch stainless prop. 3 blade.
5800 RPM top end runs about 41 MPH (maybe 42 MPH) trimmed nicely. Jack plate up a bit, motor trimmed, and trim tabs (Lenco) down slightly.
3800 RPM to 4000 RPM runs around 27-28 MPH.
I ran about 82 miles yesterday (mostly around 25-28 MPH) and it ate 17.6 gallons = 4.6 miles/gallon. I'm happy with that.
I definitely lost my hole-shot, but love how quiet this engine is.