Speedo, Gears, Ratio Help!

[phraza]

Hi all,

I've done several searches and can't pin point what I need. Sorry I know it's been covered heaps of times. Basically my speedometer is displaying an incorrect speed due to larger tires.

My car is a 64 Chevy C10 Short Bed. It's got 29.1" tyres (255/70/15).

Tranny is Saginaw 4 speed on the floor. With a 350 V8 (had a standard straight 6 originally)

I'm guessing it has the standard gear ratio of 3.73.

Can someone please let me know what I need to buy to get the speedo reading accurate? What gears in the tranny to I need to adjust/buy. I'm down in New Zealand so it's a bit hard getting parts and I need to make sure they are the correct ones before buying them

Thanks so much!

Fraser