Remember high school physics? force=1/2*mass*velocity^2. As a percentage of the whole vehicle, it's not much difference. Is it measurable, yes, with the proper equipment, but theoretically, you could calculate it. In theory, more mass would help with regenerative braking, since there's more kinetic energy available, but, it takes more energy to achieve your cruising speed. Regen is not 100%, so there are always losses. As said, cruising on the level, almost no difference (depends on the changes in drag caused by the tires - offset maybe by raising the tire pressure; or the angle the vehicle sits, changing the Cd), it's only when trying to go up a hill or accelerating when it has any significant effect. In the winter, when heating is required, it might be MORE efficient, since body heat will offset some of the heating required!