A car without electronics.
`My` (family owned) car hits 27 years of age, diesel, runs 1:21 to 1:23 (around 51 mile per gallon). And it actually makes that number.
Admittingly it has no fine-dust filter. But i also believe it's far less polluting simply because it's efficiency, and a 1.3 liter engine, which is small for a diesel. It's no speed monster, yet it comfortably reaches 150km/h.
The only electronics in the car is the radio playing my mp3's.
I believe regulations lead to this 'cheating'. We had fine and pretty clean engines 25 years ago! Apart adding catalysators (whats your USA word for it) and dust filters, there's not much to improve on it. Yet, govs want to because they think and believe like economists do: that technology can always improve.
Well, it simply can't. You have to achieve a good combustion. Of course electronic timing and fuel pump control / injection control may help, but it will be very marginal. We had excellent engines without any electronics, and the added electronics will only help improving the last few percent that mechanic solutions can't.
Off-topic: this car was made by VW...