I suspect the 18/78 might be overgeared, causing heat and poor output. 14 or 16/90 will likely be closer to the motors optimum gearing, giving you increased performance. When it comes to simply plain top speed a larger pinion is usually faster, but once you pass a point you are only wasting power and going smaller can actually improve top speed. In other cases the acceleration will improve so much that the car "feels" faster even if the top speed is slightly lower.