14" Rims and Larger Tires

Ok, so I purchased a set of 14" original rims for my 65 Barracuda. I plan to put on a set of P205/75r14 tires because I want as much exposed side wall as I can. I am trying to fill the wheel well. I am not sure if this is where I need to be for my question but here goes.

My speedometer cable is already shot so I need to replace it any way, but how do I compensate for the tire and wheel differential? I know if you go larger, like I am, the speedometer will read less because it takes longer to complete a revolution. The converse is true if you go smaller. Where can I get the right gear ratios and cable for my situation?