If you ever bought a cheap speedometer like I did, chances are you had to get creative to fit it to your handlebars. Specially if you tried to fit it to the bigger bars on a race bike. Often the latches on the brackets are too short.
There are solutions on the market, but they are expensive and don't always fit the look of your bike. Plus if you could afford to spend the money, you probably just bought a more expensive bike computer with a bigger or with a rotational mount.
(example: http://rh1project.files.wordpress.com/2011/03/procarbon_computer_mount_onbar_ww.jpg)

The second picture shows what was inside the package of my speedometer.

Tools
For this project you will need:

(Cheap) Speedometer
Knife
zip ties
rotary tool (Dremel)
Speedometer magnet & sensor
(If you don't own a dremel you can always make due with a candle and a box cutter to make the holes.)

Step 1: Decide Where You Want to Install Your Speedometer

I chose to install mine on the stem of the handlebars, rather than the handlebars itself. I like to keep the handlebars free so I have more room for my hands. To install it on the stem I have to change the orientation of the mount.
Find a zip tie that is big enough to fit around either the handlebars or stem of your bike.
Plan where you will fit the sensor on the front fork, and how you will route the wire to the sensor. Like most people, I will guide the wire alongside my brake-cable.
As you can see in the picture, the mount for my speedometer was in the wrong orientation, it also wouldn't fit my handlebars since the bracket was way too short.

Step 2: Attach Zip Tie to the Speedometer Mount

Find a way to attach the zip tie, or multiple if you want to make it more secure, to the bracket. Make sure that the placement of the zip tie(s) don't hinder the placement of the speedometer in the bracket.
Drill/cut/melt holes on the underside of the bracket. I didn't have a dremel on hand, so I used a box cutter to make the holes.
Heat the box cutter in the flame of the candle to make it easier to cut trough the plastic.

Feed the zip tie through the holes on the mount and use scissors to remove the bracket, you won't be using it since it won't fit.

Step 3: Attach the Bracket to Your Handlebars/stem

Place the rubber strip that came with the speedometer between the handlebars and the mount. It will add traction and avoid scratches to the bike. Attach the mount to the bars/stem by strapping it with the zip tie(s). Make sure it fits secure, if not add another zip tie.
twist the sensor-cable down to the front fork via the brake cable.

Step 4: Install the Sensor and Magnet

Use the rubber bands/zip ties to attach the sensor to the inside of the front fork. I chose to fit it on the right side, since I used the brake-cable to route the wire down.
If you want a super neat/clean install, you can also follow the instructable to hide the sensor-cable underneath a shrink wrap (by brianbarkey).
Add the magnet to the wheel. Use a screwdriver to attach it to a spoke on the same side of the wheel as your sensor. Adjust the height of the magnet to the height of the sensor. Make sure the distance between the magnet and the sensor is only a few millimetres.

Finally: Follow the instructions included in the package to set up the computer. Set it up to the wheel-size of your bike to ensure a correct measurement of distance and speed. At this point you can also follow the instructions to change the measurement to miles or kilometres.

Step 5: Ride Your Bike

Go out and ride your bike. Find some way to check the accuracy of your speedometer. Here we have road signs which measure and display your speed, but you can always compare speeds with a friends speedometer when riding next to each other.

I wrote this instructable to participate in the bike contest. Feel free to vote for me if you like my instructable.
Thank you for reading and enjoy riding!

EDIT: add creddit to user brianbarkey for refferencing his idea of hiding the cable underneath a piece of shrink-wrap.