The diameter of the 175=23.6 in

The diameter of the 195=23.2 in

soo a net difference of less than 1/2 in.

Being that the 175's would spin slower, they, in theory would net greater fuel economy becuase your motor would be seeing less revs.

However this is only on paper.. There are way to many other variables to be sure..

For example, eventhough the 175 rotates @ slower rate, you may end up applying more throttle to get the car up to speed becuase it no longer has the proper gearing.. perhaps, to make it up hills etc.

We could go even further, into rolling resistance (the 175's should have less because they are more narrow than the 195), inertia of the tirewheel system, energy lost due to tire deflection, etc, etc etc.

What I'm getting @ is their may not really be any noticeable difference, the best thing to do is try both and record results..

Also, here is a helpful tire size calc

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