Moving along with my project and getting ready to order gears and a locker. Excursion is a V10 that’s eventually gonna get a home built S200 Turbo setup.

So was gonna go 35” & 4.88 but mocked it up and it’s gonna need some bigger meats. Looking to go 5.13 gears and a Detroit Locker in the rear.
Calcs I came out with below. Will need to go 2hrs to our lake place and make a 5hr run to the Keys.

 

In my opinion, based on my 47K heavy towing miles in our EX, those 5.13 would be my pick for 37" tires. An effective ratio between 4.30 and 4.56 is really a nice place to be with some serious weight on the hitch. I don't know how heavy your boat is, but for us and the 12K TT 10 hour tow days are a regular thing.
Your planned power adders will make it possible to run with a lesser gear but at the cost of some of that added power, and the entire reason for adding power is to be able to fully use it, isn't it?

 

Boat currently weighs in about 7K I think. I’m a firm believer in “Too much horsepower is just enough” We’ll see how weak that V10 crank is I guess once I get this thing all built.

 

I've been hauling loads with my Ex lately and haven't had any problems scooting around. 4.88, and "37" inch BFG KO2s (overrated, probably closer a 35.5" or 36"). If you plan on towing a fair bit I wouldn't go any (numerically) lower than 4.88 with true 37s. I wish I'd gone for 4.56 because of how much highway use my truck sees, but it is extraordinarily rambunctious when I hit the city streets with the 4.88.

I cannot sufficiently recommend a TrueTrac while you have that rear end open. Likewise, I love my front locker. The traction offered by the TT is fantastic. My setup has made the recent winter storms downright laughable.

 

What rpms are you turning at 70mph? I want make sure this thing will do a 4wd burnout once I get the turbo fabbed up.

 

On my EX when it still had the 32" tires with the 4.88 gears it turned 2600 RPMs at a GPS verified 70 MPH. It now spins at 2400 RPMs with the true thirty fives under it.

 

I have the V10, 37" tires and 5.13 gears, would do the same again. At times I have wanted 5.38 as they would work better with the 40" tires I would like to run in the future.

RPM at 75 mph is roughly 2800 if I remember correctly. This is obviously different than the online calculator pic you posted above.

Being in Colorado I can feel the power difference vs. lower altitude as I have taken the EX to quite a few other states. You guys close to sea level have a definite advantage.