Does anyone have a good recommendation on an odometer? I use map my run on my phone and am ready to get an odometer for my bike.n y

Also, what would you say is an average mph on road bike on flat/moderate hill 20 mile loop? I find I'm averaging 12.5 and seem slow. I am wanting to ride with some groups that say they do about 13/14 mph. I don't think I can keep do I get faster?