Originally Posted by
brianappleby
Generally it's safe to ignore rolling resistance in this type of discussion, because it's minimal (i would say negligible) compared to air resistance at high speeds, and it's again minimal compared to the force of gravity at low speeds.
~13% of total power at 25 mph for a 75 kg bike + rider on level ground
~7% at 250 W for the same rider up a 5% grade. (per analyticcycling.com)