I love playing with these to find the best gearing solution for better mileage.
I found this on a F-body web site. Any car will work. It asks for your tire size, final ratio, Max RPM, and transmission gear ratios.
Then shows you the rpms in 100's what speed you would be running in each gear. I was comteplating doing a gear change for better highway mileage in my Volvo 960. Stock the car came with 3.73 rear gears. When I played with 3.54 and 3.31 rear gear sets I found that the 3.54's were the best option of the two. The 3.54 would drop the rpms by about 100 rpms at 65 mph. The 3.31's would drop the rpms by 200. Two hundred rpms brings the rpms down too far to be a benefit.
BTW every time you click calculate it opens up a new spread sheet below the one before it. So if you change something and hit calculate you can compare the changes with the previous calculation by scrolling up or down on the page.