I have a 1985 Trans Am with a cable driven speedo, the speedometer is a 85mph type.

What is the best way to upgrade this to 120 or 140mph speedo?

 

Perhaps as part of an ECM swap, you could convert to a VSS-only scheme and use a later instrument panel.

 

Do you know if I would need a speed sensor for my 700R tranny, and if so do you know anybody who would have them?
Thanks in advance

 

The VSS from an '86 T/A should fit just fine. Make sure the rear axle ratio is correct and that the right driven gear is installed.

Another option would be a Camaro speedo head, with the VSS installed on the rear of the speedo.