Your link doesn't work.

Sheldon Brown's Bicycle Gear Calculator is a good tool for working out differences in gearing. Assuming that you have 26" wheels with 1.5" tires, a 11 tooth small cog, and a cadence of 90rpm(about the average cadence of an experienced cyclist, but faster than the average cyclist) 40t will get you 24.2 MPH, 44 will get you 26.6, and 48 will get you 29.1. Those numbers go to 26.6, 29.3 , and 31.9 with a 29" wheel with 2.0" tires.

First of all try learning to increase your cadence to at least an 80RPM average under normal conditions, regardless of whether you decide you want to increase the gearing of your bike (which could still be understandable). This will give you plenty of speed for flats and tailwinds, but you may have to accept that you either have to spin really fast on downhills or just coast much above about 25MPH.

If you want to increase your gearing, it could be sort of a pain and either cost a lot or not work super well. Modern chainrings are designed to work in sets because the ramps, pins, tooth profiles, etc are designed to help the chain shift. If you change things out, they'll usually work, but will generally slow down front shifts to some degree. There is also the issue of the maximum chainring size.

Probably the best solution would be to use a front derailleur from Shimano's trekking line--designed for city bikes and the like, mostly for the European market. They're usually designed for a 48T big ring, but have compatible cable pull with MTB shifters. Something like this: New Shimano LX FD T660 9 Speed 34 9mm Trekking Front Derailleur Sliver 31 8mm | eBay . I'm not sure that will work on your bike because your link didn't work! You have to match the speeds (9s and 10sp are incompatible), cable pull direction (top pull or bottom pull--some will work either way), and clamp size. You could try and see how a 48T chainring shifts on your current crank, but be prepared for it to potentially be poorly. Or you can just get a new trekking crank with a 48T big ring.

Or you can decide that spending upwards of $100 (and that's assuming you're doing the work yourself)about 5MPH of top end speed isn't really worth it, learn to spin a little faster (going from 60RPM to 90RPM gets you from 17MPH to 25MPH on a 26" wheel), and enjoy your bike as it is.