Water can be lifted 32 feet with atmospheric pressure of 15psi and vacuum on the other end so 1 inch of water column vacuum is very little pressure.
15/(32*12) = 0.039psi per inch * 20 sqft * 144 = 122 pounds of force
Get your car on a level surface and see how fast you can push it and how much effort it takes to keep it at a constant speed. I did a lot of these calculations when I was calculating the gearing of my electric scooter starting by knowing the torque constant of the motor which was about 10NM per amp and calculating the torque increase by the gear ratio then the resulting thrust in pounds from the tire diameter. Knowing the angle of the steepest hill in town and the total weight of the scooter and rider (me) I came up with a gear ratio to insure some acceleration up the steepest hill. It also determines the maximum speed of the scooter. What was interesting is the 250-900 watt Hoverboard that I have had no info on the motor so I ran it up against a wall pushing on a UPS spring scale and measured the force it generated as the rear wheel spun on the hallway floor. It didn't generate much force - about 10 pounds I think, which explains the poor performance of the wimpy scooter.
Hey CO ZX2, what did you come up with on your calculations? Did you use the data to try to change anything on your car?
A few weeks ago, someone was asking about aerodynamic modeling software, and was disappointed that the programs available did not take into account the compressibility of air. This thread shows why compressibility is not too important. If you were to have a spot on your car with 3.5 "H2O of pressure, that would only be about 1% of atmospheric pressure (Atmospheric pressure of about 30 feetH20). Given all the other uncertainties, 1% compression could be pretty easily considered negligible.
I've been pondering this subject with my car too. I haven't figured the frontal area yet or the rear drag area, but I do know how much I reduced my rear drag area approximatly. I printed out a straight on rear shot of my car (like the one in my garage) found the scale by measuring a part on the car and then the photo, then I made many triangles and rectangles to approximate the area of the kammback visible from that angle. Of course there is a lot of room for error and several assumptions in doing that, but I though it would be fun to approximate. I figure the kammback subtracted about 1.174 square feet of the rear drag area assuming I have attatched flow all the way to the end of the device. It may have actually reduced it more than that if the air continues to follow the shape after it comes off the end like a good kammback is claimed to do .
I'm drawn to the physics of things, it's so practical and fun . I will do some research on the subject and if I come to an epiphany I will report back .
Best tank= 81.23 mpg on july 1st 2008
SAVE SOME GAS, SAVE THE WORLD!