I have just bought a 2nd hand bike (Specialised Tarmac SL4) which has a 50/34 chainring, the seller says it is Ultegra 10 speed but can be 11 speed (that confuses me a little!). Most of my riding is on the flat. I have a spare pair of wheels but they have an old 9 speed cassette on them from my old bike. What cassette would I be best to buy please to put on my spare wheels which will help me maximise my speed on the flat. Thanks