So I finally did a comparison of my Kickr Bike output vs Zwift on a 2 hour endurance ride on mixed terrain. Unfortunately I can't overlay the data directly at the moment, but there were some obvious differences:-

1. Zwift speed on the flat was around 6 kph faster than the bike was reporting. Roughly 36 vs 30 kph at a steady 200 W. I do think the Wahoo aero drag is a bit high, while Zwift is slippery low.

2. Zwift speed when climbing was much closer. Zwift was reporting around 1 kph faster at 16 kph and speeds were equal when climbing at 10 kph at a steady 225 W.

3. When descending, Zwift reports only half the slope gradient to the bike (with trainer difficulty set to 100%). So whenever descending, the speed reported by the bike is much lower than Zwift. I don't know why Zwift do this, but it's a well known thing.

4. The bike accelerates much faster and decelerates much slower in Zwift compared to what the bike reports. This applies on all gradients, not just descents due to the slope discrepancy above.

5. Due to the above effects average speed reported in Zwift was 10 kph higher (30.5 vs 20.5 kph).

6. The simulated bike gearing matches the Kickr bike speed as you would expect.

7. The average power reported by Zwift was much higher (183 W vs 145 W) which I expect is due to the "sticky Watt" issue whenever you stop pedalling for a few seconds. Power reported when constantly pedalling appears to be consistent. The power reported by the bike is in line with what I would typically see for a similar outdoor ride at the same perceived effort. Zwift average power has always looked high for me when riding in sim mode.

8. Average cadence was reported the same in both