When you're coasting, it doesn't matter what gear you used when pedalling. You're being slowed by air drag, mechanical drag, and gravity (if you're going uphill).
I'm not allowed to tell you how to reduce gravity, sorry about that.
But you can reduce air drag by getting your body into a lower, more aerodynamic position, and by using narrower tires if you have wide ones. Reducing mechanical drag is going to give you the least payback, but start by pumping up your tires to within 10% of their maximum pressure.