Drag for an automobile consists of two elements:  rolling resistance and aerodynamic resistance.  Rolling resistance is the resistance to motion of a wheeled vehicle due to tire friction on the ground.  Ever hear the expression thats where the rubber meets the road?  That expression came from describing the problem were learning about. 

Key Point #1.  Rolling resistance is the resistance to motion due to the tire gripping the ground under it.

Among the factors that affect rolling resistance are vehicle weight, tire inflation and composition, tire width, number of tires (trucks have many).  Secondary are those elements that affect the wheels ability to turn freely, such as brake drag, resistance in gears and axles, and even wheel bearing condition.  I wont spend much time on these elements, as they have a relatively minor effect on mileage when compared to the two major categories of drag.

If we were to list the factors in order of importance, that list would vary only slightly from vehicle to vehicle.  And I am not going to get into what the ground is made of.  Yes, there are differences in the effects of a concrete road versus a fresh asphalt road, versus an old and rough asphalt road, versus sand versus dirt roads.  Were going to assume that for comparisons sake, all the cars I will talk about are on the same type of road surface, so we can focus on the cars, not on the effects of driving on dirt roads.  I am also going to assume that the cars all have good wheel alignment and have no other components in disrepair.

Most important factors in rolling resistance:

- Vehicle weight

- Tire inflation

- How you drive (assumed equal, not discussed here)

- Where you drive (hilly or flat; assumed equal, not discussed here)

Less important:

- Vehicle speed

- Brake drag*

Least important:

- Tire tread design

- Tire tread composition

- Tire temperature

- Tire tread width

- Road composition (assumed equal; not discussed here)

- Road condition (assumed equal; not discussed here)

*Note: on brake drag: Brake pads are always touching the wheel rotors, so when you apply the brakes, they grab the car instantly, with no delay to stop the car.  1960s and 1970s cars often had significant brake drag, as the pads were rubbing the rotors quite noticeably due to springs behind the pads.  In the 80s quick take-up master cylinders in braking systems made the brakes travel more distance with the push of the brake pedal, eliminating the need for brakes to be rubbing all the time, for that quick application.  Of course, brake pads last a bit longer too with these new master cylinders.

Now you know that rolling resistance is actually comprised of the combined effects of all the above factors.  If you dont keep your car maintained by getting wheel bearings lubed, by having your differential or transmission properly serviced, or let your tire air pressure get low, you will use more gas to drive down the street, wasting gas, because your car's rolling resistance is higher. 

Also notice that vehicle speed is not a big factor in creating rolling resistance.  Rolling resistance will go up due to vehicle speed, but it does so only mildly.  The reason is that the power required to move an object goes up as the speed increases.  Of course, this additional power requires more energy, which comes from fuel. 

Anyway, I've described rolling resistance.  It comprises the resistance of rolling on the ground.  Heavier vehicles have more rolling resistance, and lighter vehicles therefore, have lower rolling resistance.  Tires inflated to higher pressures dont flatten out as much or deform when they roll over bumps.  Keeping your tires at the proper inflation minimizes rolling resistance.  Don't overinflate though, or your tire will hit bumps and potholes too hard and will damage the internal belting of the tires.  The author has experienced that, and it is not fun. 

Minimal Rolling Resistance

Cars with minimal rolling resistance are light, have two-wheel drive, have tall, thin tires that are inflated to the maximum safe inflation, use modern brake designs, and have low friction bearings and lubricants.  I would bet that a Ford Focus, the Honda Civic, and a Hyundai two-door all have low rolling resistance, for example.