Hello! You should always test on the bike you are going to train on. The choice of bike depends on your goals. If the focus is a club TT or Triathlon, you need to be executing majority of your workouts on the same bike to replicate position. These workouts should include at a minimum MAP and FTP based efforts. In most cases power on the TT bike will be slightly lower due to the closed off hip angle.
With specific regard to TT performance, it’s important to consider any differences in the power output that you can produce in the TT position (but sounds like you help 300W in your 30-minute effort), and then your ability to exceed your FTP power and sustain that level during a TT effort. Generally, we usually see folks being able to hold 5-10% higher than their 1 hour power (FTP) for a 20 to 30 minute race effort. Keep in mind that there’s not a linear increase in power demand for a linear increase in speed or reduction in time…so a 10% increase in power will yield only about a 3% increase in speed. With that in mind, reducing your drag (aerodynamic, drivetrain, and tire rolling resistance) are other areas where you can gain a bit based on equipment used (low RR tires with latex tubes; aero helmet, skinsuit, low friction bearings & chain treatments, etc.) and position changes to improve aerodynamics. If you averaged 300W to go 40 km/hr (30 minutes for 20km) and changed nothing it would take 387W to go 44 km/hr (27 minutes for 20km)…so you’re of course going to want to gain fitness, but also look to save power with aero and drag gains. There’s a nice calculator at: https://www.gribble.org/cycling/power_v_speed.html to compare some different values. Hope that helps!
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