That's a good point. I only ever reach higher top speeds when i go on long distance rides, as i increase air pressure on the tyres by about 10-15 psi.Have you checked your tire pressure? Low pressure equals a fatter contact area and more resistance, hence more energy. If you are a heavy rider, then low tire pressure will reduce your top speed even more. Think of a fat tire bicycle and compare that to the bicycle tires in the Tour de France.