Way back when, I swapped in a Dana 60 with 3.54 gears, and the truck came with 4.57 gears. The speedometer read 65 but I was doing at 80. I used the tachometer to know what the speeds were, and created a chart for when I tried a different tire size, so I knew what the new rpms were reading at specific speeds. I also didn't know about speedometer adapters back then, so there is that...lol...
