I thought other people may be interested in this kit, as I certainly am.

Digital Speedo Correction Kit on the Jaycar website link - click here

This kit should allow me to alter the signal from the electronic mazda speed sensor I am using, and correct for the different tyres (and other factors), and end up with being able to use the mazda instrument cluster with the original speedo and odometer correctly calibrated.

That would be great if I had an electronic speedo.....

Nice find Wes. I had seen the yellow box speedo recalibrator made by a company in West Woolongong NSW http://www.blackrobotics.com/yb3_calib.doc  and thought wow that's just what I need but they want $100 USD for them! So I started designing my own circuit, but now I think I'll just buy this kit & save myself the hassle of custom fabricating my own PCB.

That yellow box one would be good, but it doesn't have a big enough range of correction for my car. I need a correction of around 45-50%.

You see the mazda reads through the gearbox, and runs through a 21/25 gear ratio to the axle. Then I have a ratio of offroad tyres to mazda street tyres of about 0.8:1, so together about 0.67:1, but they enter it the other way (inverse) so about 1:1.48.

Will have to wait until the kit comes out in september I guess...