[beatr911]

Just increasing the stroke will yield a bigger displacement engine with an increase in torque, yes, but won't do anything for mileage unless you need more torque than you already have. Increased output from an engine means it uses more fuel (you knew that). Increased efficiency is usually when the engine is operating at it's maximum output for a specific RPM, wide open throttle eliminates the massive restriction of the throttle plate.

Of course for any significant stroke increase you'll need to change rods and pistons to accomodate the block it's in. Compression ratio could then be whatever you want.

Optimizing the engine for low speed operation and low maximum power output to what you'll routinely use can be done more efficiently with cam timing and intake and exhaust header length. Increased compression ratio will also help by milling the head or decking the block, or by pistons. Low tension rings help, single ring pistons help. Cam profile changes minimizing overlap will help. Potent ignition systems help. Optimized ignition timing helps. Very accurate carburetion settings or ultra fine injector spray patterns help.

Hope this helps!