I'll pop some weight in the back and see what happens.

The last time I weighted down the back of my car (enough bricks to build a small wall) the immediately noticable effect was to turn a car that rolled alarmingly round sharp bends into one that stuck to the road like velcro.

Could the difference in MPG be explained by your greater confidence in the bends, hence less use of the brakes?

Brake N.: A device for converting useful kinetic energy into waste heat.

Bree.