Well, I don't own a synchro but I thought I can make a note about mileage
in general.

Before just a few months ago I always measured about 19.5 mpg for my 84 
vanagon. I had a new set of tires a few months a go. Since then I have 
consistently measured 21.5 ~ 22 mpg. I think the tires make the difference.
Or the tire pressure makes the difference. I had about 30psi on my worn
Michellin XZX 6 ply tires. Now I have 38 psi on my new Michellin LT tires.

The new tires with higher pressure not only make steering much easier
they improved the gas mileage significantly too.