This is what happened to me. I removed the 40 over rv fuel mpg test injectors, in my 8300 dually i did 18.5 mpg all hy miles. Then i removed the 20k over hg and replaced it with a factory one and put my 7x12 injectors back in. The 7x12 are around 200 to 240 over stock. I have to drive with the overdrive off because it can get smokey in traffic. So i drove around the city/hy mostly at 2000 rpm's and i am getting better fuel mileage. The turbo is a 64/71/13ss htt.So the question is do you think the gh change helped or running the turbo in it's peak map helped ? Or both.

Update 250 miles to 1/2 tank. Thats 50 more with stupid big injectors and overdrive off. I did 200 in overdrive all on the freeway. Go figure.

removed the 20k over hg and replaced it with a factory one and put my 7x12 injectors back in.

??? Head Gaskett ?? Dunno man thats weird - even the HY would still be in its compressor map range at up to 25-30psi which your not running just driving around town. And such quick spoolup with the hy35 would think efficient at lower end So 1) More air flow with the 64/13 at the same rpm/speeds better combustion/efficiency 2) Lugging a little before in town in OD but not as obvious with the RV275 ? I more inclined to think its both like the extra airflow cfm and the runnning without OD. Injector style plays a role too ... I get same mileage with my +75 HP CPP injectors as my rv275s and both more than stock.

Thats what i am thinking the head gasket. I had plans and they changed but the 20 over headgasket killed the low end for sure.