If you go to a smaller tire you will probably pick up a few MPG because of the smaller diameter tire. There easier to turn. Now the thinner tire will have less rolling resistance because less contact on surface = less friction. It wont be to extreme but it its still true giving that both tires are the same tread pattern same surfaces, and the only difference being the width of the tire. Now if you go to a smaller tire it might be less highway friendly unless regeared to compensate.

 

If its a non driven wheel so we leave the gearing out of it. all else equal. I'm still thinking the tall tire would roll easier. not sure why. maybe my brain is auto matically thinking taller, narrower to keep the same tire patch area the same. it might be a mute question because aerodynamics may be more important than rolling resistance above 50 or so MPH. i still would like to get low resistance tires on my test mule somehow.

 

IMHO, within reason, diameter is the least important thing. Ply flexing, rubber compression and movement, and last, but not least, type of tread are much more important.

 

So why is the trend going towards wide tires? Is it just for cosmedic reasons? Narrow tires are better for rain. Tall tires for mud and snow. Wide tires are better for Bugatti Veyron. (awesome car btw)

 

Part cosmetic, part better traction and handling. They also better on loose ground, in off-road. And I learned that on the hard way.

 

same reason as some people put boom box's in there car's. But at least with the tires they don't make you deaf like boom boxes do

 

I thought I'd throw my two cents in here:

I am an aeronautical engineer and was part of some wind tunnel testing a year ago taking a look at decreasing induced drag on pick up trucks. The three settings tested were 1) open bed, closed tailgate, 2) open bed, open tailgate, 3) closed bed, closed tailgate. 1 and 2 turned out to be almost identical in drag, 3 was only slightly lower drag (about 1-2 percent lower, nearly nothing). While the maximum cross-sectional area exposed to the airflow is correlated to the amount of drag on the object, in most cases the manner in which the flow separates from the vehicle is much more important.

As air hits the front of the car, it streamlines around the front of the car and on to the sides. As the air moves towards the back of the car, it tries to stay "attached" to the surface of the car as the surface closes at the end of the car. Inevitably, the flow will "detach" from the surface, resulting in a large, turbulent, low pressure wake behind the car. The front of the car sees a local high total pressure in the airflow. Therefore, the pressure gradient between the high pressure in the front of the car to the low pressure in the back of the car is the MAJOR source of the aerodynamic drag, especially at highway speeds.

To mitigate this problem of the large low pressure wake, the surface of the car must be design to get the flow to stay attached as long as possible before separating: reducing the size of the wake. An example of this principle is in a Porsche: the back of a Porsche is typically sloping from the roof all the way down to the bumper. The car sort of looks like half of a teardrop with the tapered end towards the back. The tapered rear encourages the flow to stay attached, reducing the wake size, reducing the low pressure, which reduces the drag on the car.

Consequently, it is quite easy to see why SUV's and pickup trucks get poor fuel economy: they are very draggy vehicles! (and they have large displacement engines) The backs of SUV's and pickups end extremely abruptly, leaving a gigantic low pressure wake. To fix this, the rear must be sloped from the top of the car down to the bumper. For a pickup to be a pickup, it MUST have an open bed, therefore a sloped bed is simply not applicable, which dooms pickups to lots of aerodynamic drag.

Surprisingly, the amount of "roughness" (ie: bumpy protrusions, messy undercarriage, mirrors, door handles, etc.) is relatively small compared to the pressure drag on the vehicle. I don't mean to minimize the importance of reducing roughness, it does contribute to drag, but not by much when compared to the high-low pressure gradient.

Hope this sheds a little more light on the subject. At the same time this tunnel testing was going on, I did my testing on comparing the drag generated between the racing corvette and Ford's new GT. To my pleasant surprise, the GT produced more downforce per unit drag than the 'vette. Way to go FORD!

 

Excellent info Erik. Have you seen Dave Whitmer's truck?
http://powerstrokenation.com/photopo...er.php?uid=175

Also have a 91F250 that i am going to dedicate solely for aero ideas. So what i need is a formula that i can input length, width, and height of the brick, i mean the truck, and a value along the front to rear centerline of the truck and it will tell me how far out from that centerline i should be with the surface of the tear drop to be optimal aerodynamically. Also check out my gallery "windystar" pic 3 and tell me what i did wrong. John

 

so if we got rid of most of the cab to decrease frontage area and drove in couch potatoe mode we would decrease drag and save gas! i like that idea, i don't know about you but I'm kinda partial to the couch potato position anyway. And thanks for the info, very well said. Dosent the sharpe gradient cause the rear of the truck to lift up as speed increases with less load transfered to the ground as well?

 

That's a pretty good start. The more you can continue to slope those those rear extended panels inward, the better. Obviously, though, I assume you still want your rear hatchback to be usable, so you can't cover the rear completely. But if you could extend the panels farther and continue to slope them in, that would help.

 

So what your speculating is whether the low pressure/high pressure difference between the top side of the rear of the car, and the undercarriage near the rear of the car, generates lift--> reducing ground force on the wheels. In short, the answer is Yes, that doesn happen, and that is why Porsche puts a small spoiler over the rear of their car. That spoiler is actually doing two things though: providing additional downforce, AND delaying the seperationg point of the airflow over the car. Remember, the longer the flow stays attached, the smaller the wake and the smaller the drag. However, it must be noted that the pressure difference between the top of the car and the undercarriage is a small difference, especially when compared to the pressure difference between the front of the car and the back of the car. In a pickup, there is no surface in the rear that keeps the flow attached: the flow over the bed is a completely turbulent wake. And just like an airplane wing that is stalled, the truck bed is generating next to no lift when there is a big chaotic wake above it; the weight of the vehicle greatly outweighs any vertical force created by the small pressure difference between the top of the car and the undercarriage.

 

so we must keep the bed full with the load tapered towards the ear of the vehicle then attach to the tailgate a fairing that begins the size of the tailgate the ends in a point at a distance of approximately six feet aft to reduce drag?
Wouldnt we do better to just not redesign our trucks and just drive slower therby decreasing drag?

 

Yes, that is true. The correlation between forward speed and drag is exponential, not linear, so it definitely pays off to drive slower when possible, from a drag point of view. But since our ultimate concern is in the mileage of the vehicle, drag must be balanced with engine rpm and throttle position. What we gain by reducing drag generated by the truck is a lower amount of fuel necessary to push the truck along.

 

Nope, it's not exponential. It's quadratic.

 

it contains both the first power and second power of a variable...drag

On a lighter note:

What would a formula look like that one could input length, width, and height of a truck, and input values along the front to rear ground level centerline of the truck and it will tell one how far out from that centerline one should be with the surface of the tear drop to be optimal aerodynamically for 60 MPH, std conditions?

Or put another way how does one calculate how big a half tear drop would be to put a pickup truck inside? Guys, am trying to reach out here.