Daughter needs Gas mileage from 273 commando

The Offy is comperable to the D4B series. LD4B LD340, etc, without quite as much upper end RPM range.

Advancing the cam cycle will put RPM down a touch, depending on duration and overlap. On a low overlap/ duration, you won't see high RPM anyway. I put mine in at 3°, so it would hold above 0 longer on a double roller chain. I also greased the daylights out of it with bearing lube before installing it.

What it will do is set your dynamic compression up higher. It depends on what you're looking for in final cylinder pressure and what kind of gas you want to use.

The 273 and 318 have a niche, with closed chamber heads and a good compression distance, because they have a short enough stroke to obtain quench distance, without bringing the compression ratio into the 10s for street use on mid grade. This is one of the reasons why I elected to build a 318 instead of one of my 360s. Quench benefit and mid 9s, on a dynamic 7.8 at 5400ft

I would get that 273 nice and warm and do a cylinder pressure check on all 8 and read the plugs, before deciding on what to do next with a tune.