12.7 Detroit need a new turbo (upgrade?), hauling light and best mpg?

Its a 2001 Detroit 12.7 factory was a 430/500hp and now its supposedly set to 470 straight. Looks to be the factory gated Garrett part number 23528059 and leaks oil at the shaft out the exhaust if idling for a bit.

I haul light, total weight of truck and freight is like 30k lbs, and my current mpg loaded is about average 8-8.5 unless in the mountains then its about 7ish.
Ive tried different speeds 64-70 and all are about the same. high gear in the 13 speed isnt much rpm difference.

Without a different tune in the ECM is there a better turbo that would yield me better mpg? I dont plan on turning the power up ever. Since i have to replace the turbo anyway, i was curious if i could squeak a bit more mpg out of it somehow. What is the smallest ungated borg warner that can be ran without having to watch it and make sure it doesnt overboost? Or just replace it with the same factory one? Really trying to maximize mpgs the best i can.
Thanks.

 

Do you know what the wheel sizes are on these?
From what i can find the stock garrett is 74mm compressor wheel and a 82mm exhaust... ? thats just from some forum.
And that 171701 is a 71mm/88mm ?

Would this have similar driving/spooling characteristics as the stock one? less or more lag?
Thanks.

 

With those 2 being the same exhaust wheels, what would the compressor side differences do? Would the larger 75mm move more air faster? Less lag? The compressor wheel on the factory garret is what im not sure on the size, 74mm maybe? Im just curious the characteristics of the larger difference in size from exhaust side to compressor, the 171701 has a way larger gap, not sure if that is relevant or not. ?

 

The K31 (Borg warners direct replacement 172743) im sure would spool faster and perform better, with hauling super light a little lag probably wont hurt me, better MPG is the goal. Just curious the difference between the 171701 and 171702? drivability wise. I'll trade a little lag for mpg.

Im curious what stops an ungated turbo from overboosting? Sizing it correctly?

 

Thanks., I cant find the specs to the stock garrett anywhere to compare them to the K31 The K31 (pn: 172743) are Compressor side- Ind-71mm / exd-102mm and the turbine side ind-86mm / exd-77mm w/ 1.28 A/R.
Anyone have the specs to the Garrett 23528059 ? Really curious on the difference between it and the K31