During my restoration project on my '68 Camaro ('Ricky Camaro'), the shop and I engaged in a conversation concerning what ring and pinion ratio to purchase and install. My issue was to balance RPM at freeway speed with optimal acceleration. Given the 396 is fully rebuilt and somewhat nasty, and given I'm placing a fairly high-end stereo that will be competing with fairly loud exhaust, buffered by Lizard Skin sound dampening, we ultimately purchased a 3.42 set to place in the 12 bolt case. I sniffed around the internet for a way to determine RPM's with different potential R&P ratios, and came up with this little jewel, that provided the tools to chart RPMs in each gear. It looks like this:


Choosing the RPM formula near the bottom, it turns out to be fairly simple: RPM's equals velocity times transmission gear ratio time rear-end ratio times 336.13, with the result divided by tire diameter in inches and tenths of inches.

Given that, I created a spreadsheet that plots each gear against MPH in 5MPH increments, starting at 5MPH and ending at 120. My other arbitrary choice was to limit the 396 to the area of 5K RPM as a red-line. Probably conservative, but engines have gotten spendy.

Off to the side of the printable portion of the sheet, I have yellow-colored variable inputs for Rear-end ratio, the ratio for each of 4 transmission gears, 1st thru 4th, and finally, the tire diameter. The spreadsheet is set up to allow any of these to be changed, and once you change a single entry, the entire RPM chart changes accordingly. This makes it apparent what RPM's your engine is turning at various shift points.
The entire exercise was very interesting, and perhaps (hopefully) useful, if anyone wants a copy of the Excel sheet. Here is how it looks:



Data entered here is for my Muncie M20, and I didn't build the sheet for 5 speeds, but that would be a simple modification. The light gray numbers are just RPM's below 2K, where I see no reason to be other than in 1st gear. The darker gray areas on the right are simply used by the left hand cells to make their computation, and they automatically populate when you change the top yellow number.

It's pretty cool to view the difference changing to a 4.11 rear ratio makes, or changing the diameter of the tires. My wife says I have entirely too much time on my hands...

 

Oops, forgot to include a scan of a print-out of the spreadsheet:

 

cool...but there are several sites where you just enter tire size, trans info and pick a gear ratio and it will show rpm at a given mph you enter

but nothing wrong with rolling your own version. It's really helpful in picking a gear ratio for your intended driving use

 

Yeah, that was how I started down this road. Originally I used the Excel spreadsheet without any formulas, just to create a form similar to the scan print above, that I filled with data from one of those sites. I wanted to see shift points across the four speeds, while those sites only show a specific RPM for a given gear at a given speed. Then I noticed I was getting slightly different numbers from different sites, and most disturbingly, different numbers from the same site at different times. Hmmm. So I decided to stop depending on others and just use Excel's excellent math calculator and was fortunate to find the formula sheet I display above, that does all the conversions like inches in a mile, etc. It was a simple task to add formulas in each RPM cell. So, as long as you accurately know the tire diameter, this math doesn't get off by a mile per hour or more. Anyway, it was fun to build. But absolutely, feel free to use the various sites. I originally used this one: https://purperformance.com/p-29669-rpm-calculator.html

PLUS, using my speadsheet, I can change one variable and the ENTIRE table updates. Takes all the work out of it.