I have manual and also I have a display of current fuel consumption. When you put it in neutral while moving ( so you can roll a bit forward in traffic when not on flat road ) the fuel consumption skyrockets. I have checked with my parents’ car and it also does that.
In: Engineering
When idling, you’re using a little bit of fuel every second to keep the engine running. But your car measures its fuel consumption in terms of fuel per distance. And it updates this every so often, based on fuel consumption and distance traveled in the last, say, 10 seconds.
When idling but creeping very slowly forward downhill, you are using fuel, but that has nothing to do with how much you move. The fuel is being burned not to move the car, but to keep the engine running. Based on a quick search, a reasonable number for this seems to be about 0.6 liter per hour. That means that in 10 seconds of idling, you burn about 1.67 ml of fuel. Now say you only crept forward about 1 meter in the last 10 seconds. Your car puts those two figures together: 1.67 ml fuel spent while traveling 1 meter. It doesn’t know that the two numbers have nothing to do with each other – it just assumes that the fuel was spent to move that distance. So how does this translate to km/l? Well, 1.67 ml goes into a liter 600 times, so 1 liter of fuel at this rate would get you 600 meters of distance covered, i.e. 0.6 km/l. Or, if you prefer, about 167 l / 100 km. So this is how you can get these ridiculous estimates of fuel consumption.
When idling while stationary, the software or circuitry that calculates fuel consumption is probably just smart enough to have a rule that says it shouldn’t calculate a liters-per-km figure in that case, because that would be meaningless and might lead to divide-by-0 errors. As soon as the car moves, though, that basic fail-safe is lifted and now it thinks that it can calculate fuel consumption as liters per km (or km per liter).
The units of fuel efficiency are miles per gallon, or liters per 100 kilometers depending on your country’s measurements. I’ll use the latter because it’s easier to talk about. How much fuel is burned to travel a fixed distance?
If you are just crawling forward, how long does it take to travel 100 kilometers (around 65 miles)? Basically days. How much fuel would you burn in a few days even at idle? Many many liters/gallons – I think I estimated my car would drain its fuel tank at idle in about 2 days.
So, yeah, the efficiency of moving at a snail’s pace while the engine is running even at idle is mathematically really bad.
It depends on the unit, if it’s l/100km, you aren’t moving, but you are still consuming fuel, so it should ∞ l/100km
If it is in mpg or km/l, then there must be something else, because it should go to 0
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