[oil pan 4]

EGR appears to improve fuel mileage in most gasoline powered vehicles.
Catalytic converters on non-tubocharged gasoline engines that are working and not clogged up do not appear to hurt MPG.
My 489 cubic inch engine at the machine shop is going to have 11:1 compression and I am going to try to not run premium gas once I have everything sorted out. I am pretty sure these high compression engines can run and cruise down the highway just fine with regular gasoline. The problem is when you step on the gas, vacuum drops, filling the cylinders with air more, causing higher compression, more heat in the air fuel mixture.
There are 2 ways to prevent pre ignition. You can burn fuel that is more resistant to higher heat and pressure or you can try to cool the mixture off. There is a 3rd way to ward off detonation by having thinner air to start with. My thin air lowers adiabatic compression by about 2 full numbers. So an 11:1 engine at 4400 feet is more like a 9:1 compression engine at sea level.
Aside from the thin air I am going to try to cool the air fuel mixture down with water and methanol under part throttle and above.

Another dumb thing auto makers will do is run the idle ignition timing with little advance, that way more heat goes out the exhaust to the converter. You should be getting at least 20 degrees of advance at idle.