It's worth noting that the computers in newer cars actually make it more difficult -- or even impossible -- to do some of the things I did to the Bronco.
For example, I tend to advance the timing a few degree more than factory specifications, for better fuel economy. This tends to be ineffective in computerized vehicles because the computer typically readjusts the timing back to factory specifications.
Advancing the timing leads to earlier ignition during the compression stroke, resulting in a more complete 'burn' and 'squeezing' every bit of energy out of the fuel used. Advancing the timing means 'riding a razor edge.' There's more of a tendency to ping and knock under extreme loads and acceleration. (Going too far results in unacceptable pre-ignition and possible engine damage.)
Also worth noting is I stayed with stock configuration and OEM-type components when I rebuilt the Bronco. A large part of the improvements in emissions and economy was an effectively new engine. (The engine was a 'vanilla' rebuild, done by a machinist with a stellar reputation.) The remainder of the improvement was careful installation and a lot of small things I did, along with the use of a sophisticated diagnostics machine to 'dial in' the engine.
I expected, and did experience, some reduction in the improvements over time and use. Keeping it reasonably close to initial performance require regular careful maintenance.
I'm going 'all in' on MLP (My 'Little' Project). One of the goals of MLP is 20+ highway MPG in a Class A motorhome. (Other primary goals are reliability and durability.)
I have considered converting MLP to a throttle body injection (TBI) fuel system, with associated computer, sensors, and mechanisms. I not sure if that would be cost effective (and may be counter-productive), given I have the means and ability to squeeze maximum efficiency out of a non-computerized engine.
Important: My particular driving style, especially in large trucks, is a very significant contributor to better fuel economy.
Finally, I believe it's possible, but relatively expensive, to tweak computerized engines. There are tools and products available for reprogramming vehicle computers. These are marketed primary to people desiring higher performance, in terms of horsepower, acceleration, and so forth. It may be possible to use these to reprogram for better fuel economy.
