I got 2 cars and i want to put my slightly bigger/thicker tires on my escort... can i do this? and would this affect gas economy and speed?
I'm not sure on the exact sizes at the moment, but the nissan has rims as well.

gas economy - might take it down a little
speed - higher top end, less torque at the wheels, you will be going slightly faster than your speedo says, dependent on size of tires.

This is a different site, and for a different car, but the physics is the same, it might help you out.

http://www.miata.net/garage/tirecalc.html

The mane thing is that the rims fit with the bolts.
The spedometer will show u a wrong speed.

Dimman