Outright race bikes, especially and particularly the aero race bikes (and Specialized has kind of deprioritized the Venge, offering it as a frameset only, in favour of making the Tarmac their one race bike), have longer reach and lower stack than more general purpose models and much more so than endurance road bikes.
The Specialized Tarmac with 395mm / 565mm in size 56 actually that's the old model, new is 398/555 and it's touted as faster and a replacement for the Venge is completely normal when compared to other bikes designed with a similar purpose; the Canyon Aeroad in size M has a 393mm reach / 560mm stack, Giant Propel in their ML has 398mm / 562mm stack, Orbea Orca Aero in 55cm has 391mm / 567 mm, Focus Izalco Max 390 mm / 544 mm (or next size up, 400mm/554mm, almost identical to Tarmac SL7), 391 / 563 for Trek Madone in size 56...
Specialized is not unique. For this sort of bike, as long and low as the rider is comfortable is the idea, because it is simply faster.
It’s not even a little faster, btw, it is a fair bit faster. Recently I tried to ride a 320km (double century) with an idea that I'd attempt it in under ten hours, if possible. It ignominiously ended with a torn sidewall and the last 100km rode home in a bus, but the first 40K bit which two years prior took me 233W to go at 32.5 km/hr, this time, similar time of year and temperature and with similar weather and similar luck with the few traffic lights on the way, took me only 201W for 32.4 km/hr. That's a significant saving - there's just no way in hell I can do 230W for 10 hours, but 200W for 10 hours seems within the realm of possibility.
What did I change? Removed the 2cm of spacers and replacing a -6 degree 110mm stem with a - 17 degree 120mm stem , went with a narrower handlebar, a better quality trisuit, and a 45mm deep aero wheelset instead of the 35mm OEM alloy set. The long and low position on the bike has a big impact on speed, if you can get comfortable in it. That's why the bikes made for racing are designed to accommodate it.
Btw, Aethos is designed to have same geometry as old Tarmac, it's not an endurance geometry bike.
Last edited by Branko D; 09-19-21 at 03:37 PM.