Remember also that it may have to do with the derailleur setup. Every derailleur can only "take up" so many teeth.

That said, 48 is plenty on the front for most riding. As I was bombing down a mountain yesterday on my Surly Disc Trucker, I found myself without gear to accelerate. I was in the drops/sprinting position, my back was basically flat, and when I looked down, I realized I was going 33 mph. Its top gear is 48x12. According to Sheldon Brown's gear calculator, 48x12 is good for 35.5 mph at 120 rpm and 29.6 mph @ 100 rpm (with 26x.1.5 tires, which is as close as I could get to mine). Do you think you'd need much more than that on your flat bar bike? I don't.

I highly doubt you'll see 33 mph on a flat bar bike of any kind.