Which is better for increased MPG at highway speed without a load (i.e. in a car, not a truck.)?

 

Too many other variables. Either can be better than the other....it depends on bore, head design, cam and a multitude of other variables. Keep in mind the 7.0L Corvette Z06 has a 4.00" stroke engine and gets better highway mileage than the 4.6L Ford that has a 3.543" stroke.

 

There was a good article on bore vs stroke in one of my car magazines.

They chose a BB Chev to demostrate the debate but it was against horsepower.

Pretty easy to relate torque to stroke but the article showed very little gains in either direction since many not so obvious factors come into play when hitting extremes.

When Dodge decided to bring the V10 into trucks in the mid 90's to fill the void of having no big blocks. They needed a big engine to compete against the 454 and 460 from GM and Ford. The V10 was chosen because after 4.00" of bore the engines they tested were hard to meet emissions with.

Either way the difference is actually minor. You don't gain 100 hp or 100 lbs/ft with either extreme. The magazine test was less than 10 hp and 10 lbs/ft either way.

Personally I've always prefered bore in high performance cars. It's more room for the valves and you can make up for the lack of stroke with rpm and drivetrain.
For daily driver's or trucks. Stroke, partial throttle accleration is a joy for regular commuting.