It’s a question that I hear often, and have fun trying to answer. The obvious caveat being that speed increase varies immensely and depends on your position and equipment choices prior to getting fit.
Positionally speaking, the primary goal is to reduce your frontal area while maintaining your ability to pedal. We do this through the obvious methods such as rotating the rider forward while lowering the aerobars and narrowing the arms, and the not so obvious ‘shaping’ of the rider’s torso and head. Facilitating a relaxed, dropped head sometimes requires paradoxically wider elbows and higher aerobars. The goal is still the same, which is increased speed through better aerodynamics.
This is actually the same "fit" with dramatically different examples of "head discipline". Dropped head is about 1 mph faster.
You might have heard some version of, “Lowering the bars isn’t always faster.” That’s not an untrue statement, but it is a poor interpretation of the much more useful, “Lowering the bars is nearly always faster.” 99% of the riders I fit are not in the ballpark of an optimized back position for their morphology and goals. And then there is the 1% who are riding very low and have tested in the wind tunnel and determined that indeed, lower is slower. Which only proves that bike fitting is a dangerous place to have the 1% making the rules.
A rule of thumb I do subscribe to is, over a very wide range of typical back angles and over a wide range of typical speeds, for every 1cm we lower the bars, we remove from 3-6 watts of aerodynamic drag. In even simpler terms, if I can take an average fit subject riding at average speeds, and drop the bars down 4-7cm, we will pick up in the ballpark of 1 mph. In practice, I’ve dropped riders as much as 10cm and even raised a few up over the years, but 5-6cm is about the average increase in drop when taking a poorly positioned rider and optimizing them.
Equipment is another area where large gains can be made, and an important topic to cover during a performance bike fit. The big ones: helmet, clothing, tires, tubes, and air pressure can easily be worth more than 1 mph. Secondary factors such as frame choice, cable management, basebar selection, nutrition placement, race wheels, aero crankset, shaving arms and legs, and myriad other details could easily add up to another mph.
I hate when a blog poses a question in the title, and then skirts around an actual answer. So… 2 mph! That’s how much faster, on average, you should be after your bike fit. Even for well positioned riders, we can almost always find 1 mph through a combination of position and equipment. For those with both terrible positions and equipment choices, gains of 4, 5 and even 6 mph are not unheard of.
Ignoring the helmet, position on the left is at least 1 mph faster at the same power simply due to a lower position and dropped head. Add the helmet, tight clothing and race tires and we are well over 2 mph faster.
The bottom line is that the few hundred dollars spent on a bike fit is usually the cheapest speed you can buy. The speed gained can equal one or more years of proper bike training. For many seasoned older athletes, position and equipment refinements could be the only avenue available for performance gains, as the physiology was long ago maximized.
Even if we add a crankset, saddle, stem, extensions and a set of fast tires with latex tubes to the cost of the fit, we are still well under the average price of race wheels, while more than doubling the speed gains typically seen from a deep wheelset.
Peak performance exists at the nexus of training, technique, equipment, and execution. You have to train and execute, but we can cover your technique and equipment in a professional bike fit. Exemplary work can be done online via video and email, or live at my Richmond Virginia studio.