250 EFI Current engine is a 3.0L burns 32+/-GPH WOT. WOT is 55 to 60 with a load. All around average fuel mileage cruising and such is approx 1.5 to 2MPG.
The 225 Opti also a 3.0L on a 23' Whaler Outrage averages 3.0 MPG. Heavier boat by a long shot and not near as efficient in the water as the V-20.
At WOT I expect very little gain but the idle and midrange (cruising speed) is where the Optimax shines. I would expect it to be as good as the 4-strokes in the same hp. I know Joe in Miami installed a 150 Optimax on his cuddy but have not heard of his mileage. A lot of guys seem to be going with the E-tec which is also a direct injected 2-stroke albeit much more quiet. What do you think Spare, Skool, Ferm, Genie Aye?
Will I see 4MPG?

i'm not real familiar with the XS engines, so i have no idea. 32 GPH seems awfully high for WOT burn on a 250. My EVINRUDE was built up to every bit of a 250 and WOT burn on it was 28GPH turning it right up to around 6100. The 3.0L MERC I always thought was a bit better on fuel as we had an older carbed 250 MERC and it was pretty good on fuel burn on a 231 PROLINE. I know the OPTIMAX engines are the best right now out there for milage though for new engines, only ones better were the 00-03 FICHTS I believe. Just make sure and run a good oil unless you want to be changing out compressors and air injectors in it is what I have heard.

I think I need to answer it the same way I answered the top speed thread you started. "I think I need to drive to to know for sure" :sun:

If only you lived closer Spare. I have been to C-town once. Your turn.

I think you may be right on the fuel burn Ferm. Maybe 30GPH WOT. LOL I can tell you when I put the big prop on for top end it kills the mileage in a noticable fashion. Load the motor and the EFI just keeps dumping in the fuel. Carb motors seem better on mileage in the same hp. Big diameter smaller pitch props that can easily be turned to the 5800 limiter, like the Mirage that you suggested, gives the best fuel consumption. My propslip numbers are out of sight with the 23p Tempest because it has to be trimmed to slip to get max RPM. In the 100 degree heat, the 21p is just about perfect all around. Last weekend, I ran the 23p with three people and a fairly heavy loading and I could only muster 55 MPH out of the motor turning 5500 RPM. Plum sluggish out of the hole. We went 28 miles and burned nearly 15 gallons of 40:1 premix. Mostly cruising at 40 MPH. So 1.8 MPG average for the trip.
Started to reassemble some last night I will post some pics later.

Disassembled tuner from exhaust plate, painted 25" leg prepped and ready, reassembly started, bolted tuner back to exhaust plate, lower mounts to driveshaft housing, 25" water tube in SS, Bolted the assembly to the driveshaft housing and mounted the housing to the swivel bracket. Waiting on a few more parts.
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A few parts have arrived. Not installed yet.
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What, no carbon fiber?

Saving that for Reel's 1974 Cuddy rebuild.

I see you didn't cut the tuner. Shaun gave my buddy the dimensions to cut the tuner to make a 300 style out of it. from what we've been able to find out, they all use the same tuner, just cut to different lengths

I think the 250XS needs to be stock as Merc had it for this boat. I fear the performance mods help top end, but I need a more fuel efficient package/less noise and as dependable as I can make it for what it is. From what I can tell the tuner looked identical to the 3.0L 225 EFI tuner. It ought to have some snap, we will find out next week.