So the first thing we need to address is speed is basically power over drag. Forget about Watts per Kg, Watts per CdA (aero drag) is what really matters. The biggest factor in this is your body ~80% of aero drag is your body, and of that, 80% is the shape itself. The remaining 20% is what we can improve with clothing.
We’re also going to factor in the $ per Watt saved (roughly).
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Shaving your legs - it’s free and from a style point it’s probably something you should do anyway.
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Bike fit. Combining aero testing and bike fitting. we have a blog on how to do DIY are testing what can be found here: Aero Testing at Home: Step-by-Step Guide for Cyclists – Rule 28
Another thing to test out is your Watts/Heart rate over ~10min threshold. This will give you an idea of how sustainable the power production is. A position that for the same speed at a given power, that you achieve with a lower HR is your best option as it will be more sustainable. -
Aero Socks. These are basically the only clothing upgrade that’s guaranteed to make you faster. Skin suits can vary between people. Also, $ per Watt saves they’re great.
4 Aero bars kind of fall into bike fitting too as it’s not the pars that really make a difference. It’s the fact they’re narrower on the hoods that pulls your arms and shoulders in. For this reason if put them here but if we’re talking about aero bars in isolation they would be in last place.
5 Aero Skin Suit. After bike fit, this is likely the greatest gain in terms of raw savings there is to be had. It’s worth testing out multiple suits if you can as there is variance between riders. Cost per W is higher than aero socks,
6/7 It’s a toss up between helmet and wheels IMO as it depends what you’re upgrading from. Cost per W for wheels tends to be worse. For TT riders a larger size also tends to be the faster option if your head is in line with your shoulders.
8 Last place is frame. It’s very expensive for top brand vs. an entry level. It’s much more important that the frame allows you to achieve a riding position that works for you.