Currently segments like city cars and commuter cars use the same economy test (?), the main generic/combined fuel consumption. Maybe you could provide city and highway values as well (or instead), and apply these to the appropriate segments where economy is directly part of the desirability score. At the moment, hypothetically, a city car could have very good economy even though the economy is obtained through high efficiency at cruise (‘highway’) rather than during acceleration (‘city’), which would be the more heavily weighted economy in reality.
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It would be a tough but interesting calculation. Higher gear ratios will kill your highway economy but can improve city economy. (I read offroad magazines occasionally)
Can confirm with my own truck. Barely have to use any throttle to row through every gear, so I do fine in the city (Relative to the fact it’s a pickup on 33"X12.5" tires). The highway leaves me spinning at about 3000RPM however, which doesn’t help the efficiency situation at all. 
You can see fuel burn rates at different speeds in the fuel economy tab of the overview. It’s not in a simple mpg or L/100km but it’s fuel burn at 5 different speeds and you can work out the rate in a minute or two. A city cycle with stop and go traffic and lots of small accelerations would have to be programmed separately And there’s really no reason to.
Which is why separate city and highway economy figures don’t exist in real life, right? 
Knowing these figures isn’t for personal satisfaction, but to enrich the desirability game of cars that require some sort of economy. Perhaps the current algorithm is way more complex than it appears, but this seems like a low effort modification to make two tests where one is weighted to lower cruise speeds and more acceleration events for city, and vice versa for highway. Depending on engine design and gearing, city and highway economy figures can be quite different and not precisely indicative of one-another, and some segments clearly prefer one economy over the other.
Right now as I said the game does several steady speed fuel burn calculations, it gives a combined number because it’s a single score to compare across all the regions. If in the future the developers add a separate city and highway score somewhere it hasn’t been mentioned where I’ve seen it.
That’s killrob talking about the current combined cycle.
I know this isn’t a part of the game at the moment, hence posting under suggestions.