What would happen if I did that? ;-p
EDIT: .8c relative to what? The ground observer? I think I answered too quickly without thinking this through. EDIT (2): OK, I thought about it some more. Since there is no acceleration going on, the two frames are equivalent. The speed of the car as measured by a ground observer is the same as the speed of the ground as measured by the driver. Has to be - both frames are inertial and neither frame is the master reference. So, if the ground observer sees the car pass at .8c, then the driver sees the earth pass at .8c. They will both see time and length differently in the opposite frame than the other observer, but they will both see the opposite frame passing at .8c. The speedometer is just the way that the driver measures the speed of the passing earth. It should read .8c. I think that's right.
According to my chart, 3/4 schedule 40 steel pipe has a working pressure of 2000 psi and a burst pressure of 12,900 psi. Reduce those amounts by 20% to allow for fittings.
