All modern cars have two useful indicators regarding fuel consumption: km left until you have to refill, usually indicated by a "⛽" followed the km value, e.g "⛽ 325", as well as a value for average fuel consumption with respect to distance. In some places this is measured in L/100 km, and in others in km/L (as well as their respective imperial counterparts)

So, anyone who has driven their fair share will know that average fuel consumption gets better when you doing long trips with a sensible speed and really goes downhill when one gets stuck in traffic. This directly gives the impression that being stuck in traffic is really bad for your fuel consumption. This has some truth in it, but I am willing to bet not everyone fully grasps what is happening (at least that's what I hope, I didn't grasp this for a long time).

The idea

Let's begin with this realization and we will slowly expand on it:

The values your car shows are related to range, not time. But especially for city driving, this is not how drivers think about their trips.

I work about 30km from where I live. This takes about 40 minutes on a good day, and more than 80 minutes on a bad day. The distance is the same, but the time I spend on a single trip can vary dramatically. So, when I fire up Google Maps, what I always look at, is the time the trip will take. But the car does not tell me if my fuel will last this amount of time.

I would like an indicator that instead of "⛽ 325", says: "more than 12 hours of gas in current driving conditions" and instead of 7L/100km says: "3 L/hour". Especially for city driving, these are much more understandable values. So, let's build them!

The car already computes the "range" consumption. Let's stick with the 7L/100km value.

7 L/100km = 0.07 L/km

So, let's say we are on a trip, going with 100km/h. Then we can compute the consumption per hour

100 km/h * 0.07 L/km = 7 L/h

My car has a 41 Liter tank, so we can also compute the time it takes to empty it out:

41 L / (7 L/h) = ~ 6 hours of fuel

Now, let's get us stuck in traffic. An average speed value usually used for traffic is 30km/h. Since we are stuck in traffic, let's also make it hard for us, with a despicable consumption of 10 L/100km

30 km/h * 0.1L/km = 3 L/h (wow nice!)

41 L / (3 L/h) = ~ 13.5 hours of fuel

It's clear that we can be stuck in traffic for much longer that we can be traveling. The range-based consumption hides the fact that the engine "sips" less gas in the same amount of time, because it frames everything around km traveled. I argue that this is not helpful for city driving.

Bonus

Another problem the L/100km and km/L values have is that when you have chosen to be shown the momentary fuel consumption (not the average), it can't really show it, because it's undefined, so they tend to either show "---" or "0".

This is fixed with the time-based consumption, as it can definitely show what the consumption is when idling, which is a nice bonus!

Final takeaway

None of this means traffic is “good” for fuel consumption. It is inefficient, both per distance and per trip. The point is that it is far less threatening in terms of fuel endurance than the dashboard numbers suggest. As a person highly anxious for this stuff, realizing this puts me at ease.