Undergrad Car low average speed vs aero drag relation

[Jurgen M]

I drive car only at country roads allways at speeds 100-120km/h, no city and no idle time-heating engine etc.

Why computer allways show very low average speed , 40-48km/h?I allways have feeling that this speed is too low because I allways drive way faster then this. Indeed all my friends have low average speed, max 55km/h.

Math says this is same that you drive car at 40km/h all the time,so average speed implies that aero drag is not importnat, because drag is irrelevant at 40km/h speeds.

So how can we describe average speed/aero drag relation, can we conclude that car with low average speed don't need good aerodynamics?
Math says yes, but my logic say no, because I drive car mostly from 100-120km/h..

Is anyone here with average speed 100km/h or more?

 

[Motore]

First of all what average speed? Of a trip? Since your last fuel refill? Since the first time driven (when the car was bought)?
My car measures average speed for a trip which is usually 900km, this roughly translates to 14 days (I reset trip when I refuel). Of course I cannot drive 900km for 14 days at 100km/h, so it's logical that the average speed will be way lower than that.
Average speed is usually computed with distance over time, I don't see what aero drag have to do with this simple calculation.
Have you tried reseting the average speed counter when you start doing 100km/h and read the average speed before you fall under 100km/h?

 

[jack action]

Math says this is same that you drive car at 40km/h all the time,so average speed implies that aero drag is not importnat, because drag is irrelevant at 40km/h speeds.

So how can we describe average speed/aero drag relation, can we conclude that car with low average speed don't need good aerodynamics?
Math says yes, but my logic say no, because I drive car mostly from 100-120km/h..

No, it's not the same. If you drive 80 km/h for half an hour and wait at a red light for half an hour, you will not get the same energy spent as if you would have driven 40 km/h for an hour. It requires 4 times the amount of energy:
$$\frac{E_{80}}{E_{40}} = \frac{F_{80}d}{F_{40}d} = \frac{F_{80}}{F_{40}} = \frac{80^2}{40^2} = 4$$
And that is assuming the car is not idling and consuming fuel when at rest.

 

[Jurgen M]

First of all what average speed? Of a trip? Since your last fuel refill? Since the first time driven (when the car was bought)?
My car measures average speed for a trip which is usually 900km, this roughly translates to 14 days (I reset trip when I refuel). Of course I cannot drive 900km for 14 days at 100km/h, so it's logical that the average speed will be way lower than that.
Average speed is usually computed with distance over time, I don't see what aero drag have to do with this simple calculation.
Have you tried reseting the average speed counter when you start doing 100km/h and read the average speed before you fall under 100km/h?

My average speed is allways about 40,50km/h, last 800km,last 5000km or last 10 000km..
I didnt try that,but speed must then be above 100km/h..

No, it's not the same. If you drive 80 km/h for half an hour and wait at a red light for half an hour, you will not get the same energy spent as if you would have driven 40 km/h for an hour. It requires 4 times the amount of energy:
$$\frac{E_{80}}{E_{40}} = \frac{F_{80}d}{F_{40}d} = \frac{F_{80}}{F_{40}} = \frac{80^2}{40^2} = 4$$
And that is assuming the car is not idling and consuming fuel when at rest.

Yes that make sense.

 

[Motore]

My average speed is allways about 40,50km/h, last 800km,last 5000km or last 10 000km..

My car has also the average speed from when it was bought till now, which is about 53km/h for 170000km. And it won't change much, because I use my car for driving in cities, conutrysides, when there is congestion, road works, highways, ...
To me it seems logical that it will settle around a particular average speed.