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Doing a little comparison I found the following information:
195/50 15 vs. 185/70 14: 185 tire will run 6.71% slower than the 195 tire at the same speed. That means that the 185 tire will show only 56.2 mph when the car is moving at 60 mph. So if you want to be travelling at 70 mph, your speedo should only read 65.6 mph.
Checked tire weight on Tire Rack and the selection is pretty meager for those sizes. Seems like tire weights are fairly comparable - so unsprung weight should stay close to the original size.
I did however notice that you can get a Bridgestone B381 in the 185 size. If I remember correctly this is one of the tires that did very well in a tire comparison in terms of rolling resistance.
So with all that in mind, you should be well on your way. I would experiment with some "drafting" to get you past the 50 mpg. By drafting I mean, following a larger vehicle at a distance you feel safe and comfortable with.
Sometimes I like to leave a larger gap, speed up just before a downgrade and then release the accelerator in gear and let the engine brake the car while catching the vehicle in front gradually. That way I can squeeze maybe a few miles of "fuel-injection-off coasting" while still having all ancillaries (A/C, power steering and brakes) operational in case of an emergency.
I'm curious to see other comments on this matter, as I may be going in a similar, but less drastic, direction. (205/55 16 current, 205/60 16 proposed.)
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