"Plus-sizing" Tires But Maintaining Original Width

[Tesla220]

Get hold of some olde dude at a trans shop. The gears that run your speedo cable are easy to replace. It just would take someone to know which ones you need to switch to. The "Driven "gear is in a housing that the cable screws into. That just pops out of the "tail shaft" on the trans. The "Drive" gear however requires the tail shaft to be removed in order to replace it.
I guess another way would be to contact a speedo repair shop and maybe they can calibrate your existing speedometer.