Current OAT is crucial for calculating true air speed from IAS and viceversa.
AFAIK, careers show only temperature at ground level, and I've seen an OAT gauge in the He 111 only. Also, only the Me 262 has a TAS gauge.
Are there any other ways to finding OAT or TAS from the cockpit or without looking at the map?
EDIT: if map icons are enabled, finding out ground speed shouldn't be too difficult. IIRC, each grid square is 10x10 km, so you just have to time it and do a bit of math and conversions. But that's a bit cheaty, and some multiplayer servers don't have map icons enabled.