Speedometer cable length?

I'm Going to install an autometer mechanical speedometer in my 70 Duster and need to buy a new cable to fit the gauge. Does anyone know what a good length to use is? The supplier has them in various lengths. He says the most popular are 63" or 83" I can crawl under the car and do some measuring but I'm hoping someone has already done it. Thanks

AlaskaJeff,
What supplier are you using? I'm looking for a speedo cable with the correct mount for my aftermarket speedometer. Thx