what is my gear ratio

can anyone tell me what the gear ratio is in a 69 Valiant 100 slant 6 from the factory. I am trying to figure out what speedo gear I need in it and this is one rearend I have not had to go in yet. When I am running 45MPH in my truck, the Valiant is showing 51MPH so I guess I need a gear with more teeth then to slow it down. Right. It has 165/80/13 tires on it so according to the chart it is about 24" in diameter. I am thinking a 30 tooth but need the ratio to make sure. This is just a plain granny grocery getter car. Thanks