Is there a conversion for in/hg to psi (-psi)???
 

Just curious if there is a formula to determine how many in/hg are equal to - 1 psi.

1 in of Hg is equal to 0.49 psi.

At 1 atmospheric pressure (1 bar), it is 76 cm or 30 in of Hg and that is equal to 14.7 psi.

not sure if it has what you need, but here is a good conversion table in a downloadable guide, if it doesn't have it you can choose the custom tab and make one that fits once you know the values

http://joshmadison.com/software/convert/download.php

So then the air in my tires is actually closer to 50 PSI or 50PSI - atmospheric pressure (14.7) = 35 PSI that my tire gauge shows?

You are right.

There are two types of pressures.

1) Gauge pressure. That 35 PSI in your tire is the gauge pressire.

2) Absolute pressure. The actually (true) pressure in the tire is actually 35 + 14.7 = 49.7 PSI so it is close to 50 PSI.

The reason that 14.7 out of the 49.7 does not "count" is because there is the same 14.7 PSI pushing in the tire from the outside. The outside 14.7 cancels out the insuide 14.7 so the net pressire is only 35 PSI. That 35 PSI is what supports the car.