We own a 38' Taiwan Tub with twin 225s. Haven't used it as much as we would like, it's currently for sale, but we have made several trips of about 30 nm. We have Floscan instruments and I did a spread sheet of mileage, speed and nmpg. We have done as good as 2.88 nmpg at 7 knots to as low as 1.45 nmpg at 7.3 knots. Big difference being wind and current. Last time out we did 1.57 nmpg at 8.4 knots going and 1.79 nmpg at 8 knots coming back. This is generally running at 1320 RPM. There is one trip at 3.49 nmpg at 5.6 knots. We had a boat following us in a dense fog and kept having to slow down to wait for them. The speeds are the average based on total time and distance.
Hope this helps,
(buy the boat and I'll throw in the spreadsheet)