On the end leg of this road trip. Currently dining with SWMBO in the Tonopah casino. And as I've been driving these highways I've been wondering. The highways in Oregon are DEEPLY rutted. The result is two rough, cheese grater ruts on the sides of each lane and a smooth hump in the middle and smoothness on the very outside of each lane.
So for fuel efficiency, which is better? Ride in the rough ruts and straddle the hump in the middle for less airflow under the car. Or, ride on the smooth humps for less rolling resistance. I haven't tested it yet. But would like to.
Whatever needs the least steering correction. That means less loading against the tires and it also means a little less drag from the power steering system.
This is my unscientific gut feeling, backed by nothing more than a vague idea and years of biking.
each bump is not only an impact, but more force against the car you have to overcome
A bumpy road means less contact with the asphalt, which means less friction which means better fuel efficiency!
Like the reverse of a knobby tire except the tire is "smooth"
If it's raining stay out of the ruts so you aren't constantly working to displace water. I'd guess the Knurled is correct in the dry.
What's easier on a bicycle - riding down a nice smoothly paved bike path, or a bumpy trail? There's your answer. 
ebonyandivory wrote:
A bumpy road means less contact with the asphalt, which means less friction which means better fuel efficiency!
Like the reverse of a knobby tire except the tire is "smooth"
I disagree- a wheel will travel farther on it's own on a smooth road. Just like a smooth tire will travel farther than a knobby one- which can be seen when you switch your bike tires from knobby off road to smooth on road tires.
Knurled wrote:
Whatever needs the least steering correction. That means less loading against the tires and it also means a little less drag from the power steering system.
This is my unscientific gut feeling, backed by nothing more than a vague idea and years of biking.
Makes a lot of sense to me.
So the consensus is that the amount of air under the car means practically nothing (at street legal speeds)
Some FE data on road surface in section 4 of this report Link
Cliffs Notes, Bumpy = bad
Different textures of road surfaces influence fuel consumption for passenger cars by up to 10%.
Trackmouse wrote:
So the consensus is that the amount of air under the car means practically nothing (at street legal speeds)
It does have an effect, but in terms of trading off a small difference in air under the vehicle vs other road factors, it's probably not the biggest effect.
In reply to alfadriver:
I was joking with my response
BrokenYugo wrote:
If it's raining stay out of the ruts so you aren't constantly working to displace water. I'd guess the Knurled is correct in the dry.
Being out of the ruts is also probably the smoothest, too. At least it is around here.
10-4 on water. I can actually feel the extra resistance when driving in the rain vs. not in my Mazda.
