It sounds like the bike is moving fast enough that you can't spin the pedals fast enough to keep up. A higher top gear would help with that. One thing to keep in mind is that if you almost never go that fast, you might be better off just coasting and then you can keep all of your current gears. One other suggestion is to look at toe clips or clipless pedals, if you don't have them. You can spin a LOT faster if your feet are connected to the pedals.
God bless!
Wayne J.