The bike will be able to go faster than 16mph on flat road. The gearing should be good for around 25mph with a brisk cadence. It really depends on how fast you are capable of pedaling, and if you want to try and go faster.
If you use the top gear and only go 15mph, your legs will be turning pretty slow, which isn't recomended, especially since you have arthritis. Keep the pedal effort light by using the easy gears.