Originally Posted by
camper8
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
Appropriate 10 speed cassettes for road riding are
11-12-13-14-15-16-17-18-19-21
12-13-14-15-16-17-18-19-21-23
13-14-15-16-17-18-19-21-23-25 (or 26 for Campagnolo riders)
50x13 is a 30+ MPH cruising gear and 40+ sprinting which is plenty, so it's about having a low enough gear to make it over your hills at a reasonable cadence, when you need to change rings, and what the chain line is like in your most used gears - 50x21 will be quieter when that's the third largest cog than when you have a 21 big cog.
You might run an 11 starting cog with a 34 ring so you can stay on your small ring longer with terrain that's not quiet flat, and either the 12-23 or 13-25 with a 53-39 crank based on whether you spend more time in small ring x smaller cog or large ring x larger cog.
Gearing is about terrain, rider weight, power output over durations of interest, and personal preference which vary a lot between people. You'll do better figuring out what works for you over taking advice from internet forum denizens.