Ratio Worksheet

[stevj]

I was digging around old files and found this Excel file I made for some research on an old Dodge Dakota I used to own.
I modified it for the 2019 Ram and the 8-speed transmission and here's what it does.
In the top section, input your tire size, speed and axle ratio and you get wheel diameter, tire RPM and Driveshaft RPM at 60 MPH.
In the bottom section you'll find a table that computes engine RPM based on vehicle speed, and transmission gear and ratio.
Remember 6th gear is direct drive in the 8-speed.

All this seems kinda pointless, but is actually useful if:
You're undecided which axle ratio to get to keep your engine RPMs in a certain range.
Or want to know the effect of tire size on engine RPMs at a certain vehicle speed.

I tried to keep it simple. I also included all 3 stock tire sizes in a small table off to the side, just for reference purposes.
All the gray background numbers are INPUTS. Change them and watch the blue numbers change.
The green background numbers are the OUTPUTS of the Tire Size, Speed and Axle Ratio, but they also feed the table at the bottom so engine RPM can be computed.
In the table, the blue numbers are OUTPUTS of all those other numbers about the sheet, and transmission gear ratio and vehicle speed.

If you've ever wondered about such things or are just curious how these things interact, have a copy and play.
And for a real mathematical thrill, check out some of the formulas used for the blue numbers.
Enjoy,

Steve

EDIT: I can't upload the Excel file - apparently not permitted, so go HERE to get it.
Here's a screengrab.


Steve

 

[Copperstate]

I was digging around old files and found this Excel file I made for some research on an old Dodge Dakota I used to own.
I modified it for the 2019 Ram and the 8-speed transmission and here's what it does.
In the top section, input your tire size, speed and axle ratio and you get wheel diameter, tire RPM and Driveshaft RPM at 60 MPH.
In the bottom section you'll find a table that computes engine RPM based on vehicle speed, and transmission gear and ratio.
Remember 6th gear is direct drive in the 8-speed.

All this seems kinda pointless, but is actually useful if:
You're undecided which axle ratio to get to keep your engine RPMs in a certain range.
Or want to know the effect of tire size on engine RPMs at a certain vehicle speed.

I tried to keep it simple. I also included all 3 stock tire sizes in a small table off to the side, just for reference purposes.
All the gray background numbers are INPUTS. Change them and watch the blue numbers change.
The green background numbers are the OUTPUTS of the Tire Size, Speed and Axle Ratio, but they also feed the table at the bottom so engine RPM can be computed.
In the table, the blue numbers are OUTPUTS of all those other numbers about the sheet, and transmission gear ratio and vehicle speed.

If you've ever wondered about such things or are just curious how these things interact, have a copy and play.
And for a real mathematical thrill, check out some of the formulas used for the blue numbers.
Enjoy,

Steve

EDIT: I can't upload the Excel file - apparently not permitted, so go HERE to get it.
Here's a screengrab.
View attachment 21085

Steve

How confident are you that those are the correct ratios for the transmission? Everywhere else I have looked, first is 4.71, and they vary from there.

I saw on one website it lists both sets of ratios, but doesn't specify which vehicles get what.

 

[stevj]

How confident are you that those are the correct ratios for the transmission? Everywhere else I have looked, first is 4.71, and they vary from there.

I saw on one website it lists both sets of ratios, but doesn't specify which vehicles get what.

That information is directly from my copy of the FSM. To date, I have seen no other reliable data to refute those numbers.


Steve

 

[Copperstate]

That information is directly from my copy of the FSM. To date, I have seen no other reliable data to refute those numbers.
View attachment 21215

Steve

I'd say that is pretty reliable then. Thanks! That changes my calculations a little bit then.

I assume the 8 speed going in the 2500 is the same then. What is interesting is they are advertising a 51:1 crawl ratio.

51/2.64=19.318
19.318/4.10= 4.71.

2.64 and 4.10 being the low range and differential ratios. Weird.