Jerry Danner’s C900 Gear Ratio to Road Speed Calculation Spreadsheet

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Jerry Danner’s C900 Gear Ratio to Road Speed Calculation Spreadsheet

On January 30th, Jerry Danner (famously of Mile High Automotive) created a post on FaceBook that began…

How fast are you traveling at a given RPM on your 1991 C900T manual 5 speed transmission?

Jerry is known within the SAAB community as being extraordinarily generous with his time and expertise. There seems no limit to what he will go through to help someone else, and this is yet another example.

Jerry offered to anyone who asked to share the spreadsheet that helps him calculate the road speed of a Classic SAAB 900, depending on gear ratio, by entering some basic values. That sounds really interesting, and useful! It also sounds familiar, since corresponds to my spreadsheet that does the same thing for the V4 and two-stroke transmissions, that I created and shared about 6 years ago.

FaceBook is terrible at long term communication and sharing. It shares something for about 17 seconds, offers a couple of revisit opportunities if you happened to interact, and then that thing is relegated to the long lost past, never to be seen again until it magically resurfaces as a “memory” (for the original poster only) every anniversary from then on. But basically, you can see a thing once and then good friggin luck ever finding it again. Knowing this, I offered to host Jerry’s tool here on at-speed.com as a service to the community. At-speed.com has been around for over 20 years and will be here as long as it remains useful to at least one person (even if that one person is just me).

Below is a Ctrl+C, Ctrl+V of the post he made, with the pictures he included. All images are credited to Jerry. I’ve fixed the rare typo and done some simple formatting to improve readability. The download link for his spreadsheet is HERE and also at the end. Enjoy, make good use of it, and let us know if you have any questions or comments. -STEFAN


Jerry Danner himself

How fast are you traveling at a given RPM on your 1991 C900T manual 5 speed transmission?

This question came up on FaceBook recently. Well, here’s your answer. I created this spreadsheet and it works. It is just a matter of math. I created a spreadsheet for a few transmissions to figure this question out. It started when Phil Quintana, who has a 1994 C900T manual transmission, where he wanted Type-9 gears installed in the primary case. It is like overdrive where you are still in 4th going 75 MPH and still have 5th. I also created this for the pile of gears I have accumulated. If I have a cluster shaft where the teeth on second gear has X amount of teeth, then I need a second gear with X amount of teeth that match the chart.

This example (below) is my 1994 C900T 5 speed manual transmission. When I replaced my synchronizers and pinion bearing. I counted all the teeth on all corresponding gears. If you have a transmission apart, count the teeth, write it down and make your own chart. This could be any transmission, bicycle, or motorcycle, it is all the same.

This spreadsheet is interactive where I can change the engine RPM and it refreshes itself. Bottom of chart is a quick reference for set RPMs.

DISCLAIMER: This example may not work for your car. Saab has many variants of their 5 speed manual transmissions. Tire size will make a difference also.

Follow this through. All teeth inside a transmission have two components. A drive gear and a driven gear. Count the teeth, log the numbers, and do the math to find out the ratio for those two gears.

Jerry’s Example

Jerry’s explanation of the above example:

– Engine RPM 2,700.
– Primary Gears Type 6 with drive (top) 32 / driven (bottom) 25 teeth.

  1. Divide 35 by 25 = 1.280. Which means at 2,700 RPM going through the primary chain gears the bottom primary gear is rotating at 3,465 RPM. (2,700 X 1.280 = 3,465).
  2. Take this answer of 3,465 and multiply it by the next set of gear ratios depending on what ever gear your you are in to get the transmission out put RPM to the ring & pinion.
  3. Take that answer and multiply the gear ratio of the ring and pinion and you will get your axle RPM at whatever engine RPM you are driving through for whatever gear you are in.
  4. In this example the axle output RPM in 5th gear is 942.55. (Engine RPM of 2,700 X final gear drive ratio .349 = 942.55).
  5. Measure the circumference of the tire by aligning your car where the valve stem is directly on the bottom. Put a mark on the ground. Push your so the tire rotates one revolution and place another mark. Take a tape measure the distance and that will be the circumference of your tire for what ever size you have on your car. In this example the tire circumference is 75.25”.
  6. Take the axle speed of 942.55 X 75.25” (Circumference of this tire) = 56,552.73 and you get the number of inches the tire travels in one minute.
  7. Take that answer of 56,552.73 X 60 minutes in an hour = 4,255,593, the number of inches the tire travels in one hour.
  8. Take that answer of 4,255,593 divide by 12 (inches in a foot) = 354,632.72 feet traveled in one hour.
  9. Take that answer 354,632.72 divide by 5,280 (feet in a mile) = 67.2 MPH.
    Here is your MPH answer.
    Very accurate.
    Hope this helps!

As promised, here is Jerry’s spreadsheet.  spreadsheet

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