speedo gear

I need a little help. My 71 duster340 has 28" tall rear tires with a 3.91 gear.How many teeth or what color gear do I need. The car hasn't had a seedo cable since I've owned it. The gear I took out was a green 34 tooth. my year one catalog shows a chart and says the 34 is for 3.23. The chart also says I need a 40 tooth orange gear for 3.91. But it doesn't show all tire sizes(height). Any info on this will help. Thanks Mike