Hi

When planning a flight fro Denmark to Marocco in a Cirrus SR22T with indicated speed 170kt I get data for Best altitude. If I change to a Cessna 172 with indicated speed of 100kt I get no data. What is the correlation between speed and Best altitude?

It's a lot more complex than that. Try actually planning the flight yourself with the Cessna at a few altitudes and look at virtual radar. You may find that its climb performance isn't enough to get over some terrain so accurate wind forecasts can't be determined for it, or similar.

Thanks Tim, that make sense, but for someone that do not know the relation between performance data and Best altitude it will be a puzzle. Any chance to create a hint why it say Incomplete Forecast?

Now I can make better planning - great!