Speedometer Pinion Gear

matthon said
So if you don't have an existing gear, like me, go by the chart, then once installed why would the math/calc be different?

255/60/15 with 3.91s = 38 tooth gear, which apparently isn't a valid gear in those calcs.

Because a lot of old speedometers are not accurate. So going by the chart might not always be 100% accurate. By the way, a 28" tire with 3.91 gears = 37 tooth per Brewer's chart.