I think there could be a tradeoff with thinner tires depending on how heavily loaded they are to begin with. One might lose frontal area for an improvement in highway economy, but the rolling resistance might actually go up due to heavier loading and sidewall flexion, hurting city economy. There's likely a good compromise in between that is narrower than most stock tires, but my point is to not take it too far. It may not provide the gains expected and could potentially be dangerously overloading the tires.