As for highway speeds, cars are calibrated to have the highest efficiency at highway speeds[2][3], so per distance, most cars will do best at reasonable highway speeds.
[1] https://news.ycombinator.com/item?id=41166749 [2] https://www.researchgate.net/figure/Speed-fuel-consumption-c... [3] https://qph.cf2.quoracdn.net/main-qimg-2ddbd8f31c84cefba540d...
Hmmm, in my experience, highway speeds are conducive to ICE efficiency and use less gas than city traffic stop and go results in. It also disperses pollution outside the areas people work/reside in.
- highway
- not highway, not city
- city
The 90-110km/h limits of regular roads tends to hit the sweet spot of engines more reliably than the 130+ of highways, a lot of smaller European car engines are uncomfortably close to redlining at 130+, especially older ones.
In my personal experience with both my current and previous cars, the sweet spot is constant speed about 65 to 80kph.
It does feel slow though.