Driveshaft RPMs

[Doug213xrj]

Hey, Does anyone know a formula using engine RPM, Transmission gear ratios and rear gear ratio to see how fast the driveshaft is spinning ?

Thanks Doug

 

[bracketchev1221]

Depends on what gear the trans is in. Assuming non OD trans, the driveshaft is spinning slightly slower than engine rpm in high gear. This is due to converter slip. Rear end gear doesn't matter.

 

[500/600]

I'll be corrected if I'm wrong..............

With a TH350:
3rd gear is engine rpm
2nd gear is engine rpm divided by 1.52
1st gear is engine rpm divided by 2.52

All with a small amount of converter slip calculated in.

 

[lilbowtie]

The rear gears have nothing to do with it, Take the ratio of the transmission gear your in and divide into the rpms the engine turning for driveshaft rpm (slight difference for tc slippage)

An engine turning 2000 rpms with a 700r4 would have the following drive shaft rpms
1st gear would be 653 rpms
2nd would be 1234 rpms
3rd would be 2000 rpms
4th would be 2857 rpms

 

[pontiac guy]

The gear ratio has absolutely everything to do with it. That's what it is. Rotations of the driveshaft (pinion gear) to rotation of the tire (ring gear) so if you have a 3:00 rear end the drive shaft rotates 3 times to 1 tire rotation. The transmission adjusts engine rpm to drive shaft speed. We can't easily go from tire speed to driveshaft speed because we don't read tire speed in. RPM. We read ground speed of the car thru the speedo. Which converts rpm to mph. Which is really a function of tire circumference and tire rpm.

Knowing the engine rpm and transmission ratio gets you there easier.

If you don't have a speedo but know all the gear ratios you can determine mph by engine rpm and tire size.

 

[Doug213xrj]

OK Thanks Guys. I think I asked the wrong question. This is what I would like to know Is how much more is the Driveshaft spinning at 70 mph with 3.42, 3.73 and 4.10 gears ? Tires are 27 or 28 inch tall. The Car is a street car with 3.42 gears 27 in tire. Im going to change rear gears to 3.73s or 4.10 and I would like to know the difference in driveshaft speed. Not concerned about Engine rpm, I have a OD.

 

[64nailhead]

I use a road speed calculator that's in excel. For some reason I can't seem to attach it to this thread (smh).

What transmission are you running?

Edit - forgot to ask why are you concerned about driveshaft rpm at highway speeds? It will spin a ton faster when you crank it up to 100+ (if you ever plan to).

 

[pontiac guy]

Divide the new ratio buy the old ratio that's the percentage. Then multiply it against the original driveshaft speed. So 3.73 is 1.09 greater than 3.42 and so on

 

[lilbowtie]

https://www.randysworldwide.com/calculators/rpm/

 

[olds307 and 403]

I personally like this one since I use a 2004R.

Reference Manual: Chapter 3: Axles: RPM calculation