Anyone Know How to Calculate Driveshaft RPMs?

Anyone have the info on how to calculate the RPMs of the driveshaft on the 6 sp auto when you change the gear ratio in the rear axle?

 

Use a calculator like GrimmJeeper and see what it shows engine rpm as in 4th. Or just drive in 4th if you want to see your current setup.

 

Typically the gear right before overdrive (D) is 1 to 1 ratio.

So 4th gear on our trucks should be 1 to 1 ratio between engine and driveshaft..

 

If you want the actual rpm you need
engine rpm, gear ratio of transmission gear you are in


Just divide engine rpm / transmission gear ratio.
If say you are doing 1000 rpm and the gear you are in is 2.94 then driveshaft rpm is 340

Now the RPM of the driveshaft doesn't change if you change rear axle ratio since the RPM is solely based on the ratios in the transmission.

What does change is the speed you are going at that RPM (speed at which axles are turning changes , not driveshaft)

 

Actually, that’s backwards. You would divide that.

 

Yep, i did have that backwards.. I have corrected.. Has to be the way you mentioned for torque multiplication in lower gears...

 

3rd gens have 6 gears.

 

These are your gear ratios in the auto so as mentioned just divide engine rpm by the gear you are in.. And 4th is indeed 1:1

First Gear Ratio 3.60
Second Gear Ratio 2.09
Third Gear Ratio 1.49
Fourth Gear Ratio 1.00
Fifth Gear Ratio 0.69
Sixth Gear Ratio 0.58

 

Assuming the converter is locked up. Dunno how that's programmed in 3rd gens

 

Software like OBD fusion will give you real time lock up status.

 

Oh yea I forget Joseph bought a 3rd gen. He had a 2nd gen prerunner for a long time

Looks like 3rd gens have 2 overdrive gears. So 4th is still 1 to 1

 

Pretty sure 4th can both partial lock and full lock, if you limit it to 4th and just cruise it should full lock. When I had the loaner I was able to accurately determine rpm.

 

Trade it in for a manual. Calculation will be easier...
Now if its the change in speed (actual vs indicated) going to a higher numerical (like 5.# vs 4.#) ratio rear axle will reduce your road speed by dividing new ratio/old but speedo will read higher than actual which is good in avoiding speeding tickets. But if you change tire diameters this also make a difference. A larger diameter tire will do the reverse since for every revolution you will travel a larger distance.

I have some slightly large snow tires I gave to a friend. With original tires his GPS indicated speed was 97 kph when speedo indicated 100. With my snows installed it now indicates 103 when speedo shows 100

 

All you need is the diameter of the tires, the speed, and the rear end ratio. Transmission doesn’t factor in at all.

 

He wants driveshaft RPM. This is solely a function of engine RPM and transmission ratios.If he is in 4th 1:1 the RPM of driveshaft will be exactly equal to engine RPM.
Nothing will change that unless you change the transmission


He didnt ask for speed or change in axle rpm which are a function of rear axle ratio change

 

he’s asking how it changes when you change the axle ratio - which has nothing to do with the engine or transmission.

Or put another way, 1000 rpm at the engine with a. 1:1 ratio in the transmission will always be 1000 driveshaft rpm, no matter what axle ratio.


The driveshaft rpm will change the same percentage as you change the axle ratio.

 

All of this discussion is exactly why I started by suggesting going to GrimmJeeper and entering the details and looking at 4th. If he wants to do a calculation to figure it out by entering different details, it is brain dead simple. Somewhere like GJ makes it easy to figure out when there are changes in tires in addition to ratio changes. Doing proportional math only works if one variable is being changed, unless you want to multiple for the gear ratio and then divide back for the added tire diameter. But that gets confusing.

The only hard part about any of that is putting in the correct tire diameter. For a 35, that is not 35". More like 33.5" on most of them.

 

Speed, tire size and axle ratio, anything else has the unknown of the torque converter (in an auto) DS RPM will be directly related to tire size and speed and axle ratio.

https://www.drive-lines.com/php10/dlrpm.php

 

Its all the same...

True tire diameter (not advertised diameter) and speed working up through the ratios, or engine speed working down through the ratios. Same answer either way

 

You

You are contradicting yourself

Yes : Or put another way, 1000 rpm at the engine with a. 1:1 ratio in the transmission will always be 1000 driveshaft rpm, no matter what axle ratio.

No :The driveshaft rpm will change the same percentage as you change the axle ratio

The rear axle speed of rotation will change not the driveshaft at the same engine RPM and trans ratio