Good question Steve - although since they don't sell the 4 cycle 90 HP Johnsons in the US anymore (only E-Tecs now), I doubt that anyone is gonna buy another one anyway.
I've never been able to get close the the run numbers C-Dory publishes - ~4.4 nm/gal with the Honda 75. With normal load, planing, I get 3.3 nm/gal. If I fill the fresh water tank,etc - it drops to 3nm, and with 1/2 water tanks, partial fuel, 1 person, etc- I get over 3.5 nm. Converting to mpg, that's ~4.2 - so it is not far off from what other C-Brats have posted. I didn't even look at economy when we were loaded in Desolation Sound. The boat was barely afloat as it was!
I suspect the extra stern weight of the J-90, along with the 9.9 4 cycle Honda kicker, dual batteries, floor pads, etc - cause the mileage to drop. But I really like EFI, and the 30 (or is it 40?) amp charger on the J-90 - it really helps. So, given what was available at the time, I'm pleased with the J-90. Today, I'd look really seriously at the E-Tec
Mac