Speedo Recalibration

I'm trying to correct the speedometer on my RoadStar Motorcycle to reflect the current gearing.  These speedos are off from the factory, but I have made it worse by adding a 31 tooth front pulley as opposed to the stock 33 (belt drive).  Now they sell boxes out there i.e. Yellow Box, Truspeed ect but these can be a bit expensive for what they do ($110.00 to $180.00).  I'm not in a position to spill the cash on that so I’m wondering if there may be a simpler alternative.

The scenario is this.  The bike has a speed sensor on the tranny that feeds to the speedo.  The yellow box for instance comes with three wires and interrupts that signal before the speedo and allows it to be changed by percentages.  My thought is that what is going on is the resistance is being changed by a percentage to the speedo.  If this is true and I know the percentage I need to change, why could I not put a resistor in place of something like the Yellow Box thus giving me the same effect but non-adjustable?

Thanks for any help,

Mutt

Hey auax,

Thanks a lot for chiming in on this.  It sounds like you know your stuff.  it's too bad we couldn't come up with a cheaper alternative that was home grown.  There'd be a hell of a market even for something fixed to a certain ration for $50.00 or so.  The adjustable boxes are just a bit pricey.  If you think of anything please let me know.  I have a market for a fix.

Thanks again,

Mutt