Speedo Healers - any alternatives?

[Joeman]

I've been messing with my gearings, and my my bike speedo now overreads.

The speed pickup is a magentic sensor that counts the revolutions of a four vaned wheel bolted to the front sprocket (see pic below) so changing the gearing for lower final drive has meant that for the same road speed, my front sprocket is now revolving quicker.

One option i thought of was to make a new sensor wheel, with less vanes, but since its only got four vanes, a reduction to three would drop the speed reading from 4 pulses per revolution to 3 pulses per revolution which i think would be too drastic change, and probably cause it to under read!

My other option is to splash out 80+quid for a "speedo healer" - a little electronic box that reduces the frequency of pulses sent from the speed sensor to the speedo.

Question for you guys is, are there any cheaper alternatives on the market that you've seen/used??

If there isnt a cheaper alternative, since im a bit of a techy IT geek, im quite tempted to dig out the soldering iron and build my own...