How fast can my bike go? I have an old mountain bike and want to change to a Pashley style girlie bike. I use my bike to go to the shops, pop to places and go out and about with my dog running along side.  I find the seating position of my old bike very uncomfortable in all areas.  My concern is, Pashley style bikes only have a max of 7 gears and I currently have 21 gears, of which I only use maybe 4.  My speedo shows that I cruise between 5 - 15 mph on my old bike but I'm concerned how fast I can go on a bike with only 7 gears.  Is there any way of roughly working this out as I've not found a shop that will let you ride a bike at full speed before you buy it.  Thanks in advance, Sarah
There are various gearing calculators available on the web. Here is one of them: http://sheldonbrown.com/gears/

The top possible speed will depend on your gearing, wheel size, and max cadence. Of course, your ability to reach that theoretical top speed will be limited by your own strength and fitness.