you gotta learn to ride a decent Cadence. Your top gear is good for 30 mph at 90 rpm(whih is a pretty good cadence to maintain for cruising along) Unless you're David Zbriskie, you can't hold30 mph for any extended period.

Even if were talking descending at 120 rpm ( a resonable cadence for a short burst downhill) your top end is good for 40 mph. Above 40 mph, the diminishing return from the effort of pedaling is so high, its usuallly makes more sense to coast.

So you don't need a higher gear. You need to learn some technique.