Try a 15 tooth cog.

That said, with 25mm tires, 42 x 16 is 18.5 mph. About 20 with a 15 tooth cog. how fast exactly are you going?

I think a lot of beginning cyclist push way too big of gears because a lot of resistance feels like it should be faster or more efficient. It isn't. On flats a cadence of around 80-100 rpms is about right. Obviously you will be outside of that range much more often on a fixed gear than on a geared bike.