This is really a curiosity thing, but how many RPMs is the drive shaft spinning at 55mph on lets say a mid 90's automatic impreza. I was thinking something with the 4.11 rear.

 

I'm not sure of the exact math to figure that out and was hoping someone here might already know.

 

Keith

Well if you say "RPMs" you should also say "MPHs"

same wrong - right?

 

Math is simple:

convert the MPH"s" to inches per minute or IPM"s"

X Miles/hour * 1 mile/5280 ft * 1 foot/12 inches * 1 hour /60 min

= Y inches/min (IPM"s")

 

Find the height in inches of the tires on the rear = H

 

The circumference of said tire is Pi ( use 22/7) times the height (diameter)

Cir = H *Pi = H*22/7

 

Divide the IPM"s" by this circumference

this yields the RPM"s" of the wheel

 

Mutiply this by the axle ratio (4:11) - as the drive shaft spins

in this case 4:11 revolutions for each revolution of the wheel.

 

this will yield the drive shaft RPM

 

Hope this helps.

Thanks to:

http://www.miata.net/garage/tirecalc.html

Using a stock tire size 185/65R15

if we use 60 MPH being 1 mile per minute @ 887 Revs per mile times the diff ratio gives us 3604 RPMs on the drive shaft? Sound right

...or...you could make a chalk mark on the tire where it meets the road...and another chalk mark on the road then slowly move the car forward until the chalk mark makes one full revolution. Mark that spot on the road again and measure the distance between the 2 marks.

Let's say the distance between the marks is 6 ft. That means that every 6 ft, the tire,(and the half shaft), make one revolution.

So....every mile,(5280 / 6), they rotate 880 times.

 

So...if you're going 623 MPH... your shafts are spinning 548,240 rph ....or 9,137 rpm.

Wow.

You guys are making it more difficult then it really is.

 

Take your engine rpm at 55 mph and divide by the gear ratio of the gear you're in.

 

Since you're probably in an overdriven gear, you will get higher rpm output (driveshaft rpm) then the engine rpm.

 

eg: 3000 engine rpm / .987 (5th gear, gear ratio) = 3039.51 rpm

You guys are making it more difficult then it really is.

 

Take your engine rpm at 55 mph and divide by the gear ratio of the gear you're in.

 

 

That's only easy if you have the gear ratios memorized and the RPM your engine turns at a given speed, which most people don't. Many people on the other hand do know their tire size, so doing it my way is actually easier, and technically more accurate to the speed being traveled.

 

Got the answer I needed though, so thanks.

You guys are making it more difficult then it really is.

 

Take your engine rpm at 55 mph and divide by the gear ratio of the gear you're in.

 

Since you're probably in an overdriven gear, you will get higher rpm output (driveshaft rpm) then the engine rpm.

 

eg: 3000 engine rpm / .987 (5th gear, gear ratio) = 3039.51 rpm

I like that way Josh. I'll keep that in mind. Unfortunately, I don't know the gear ratio of 4th + lockup on my 4EAT :-p

http://www.main.experiencetherave.com:8080/subaru_manual_scans/1990_Legacy_Owner_Manual_Scans/117.jpg

 

The 90 info should be the same for the 91 Legacy.

 

As for lockup....once locked up (like driving down the freeway) , the engine rpm should match the input shaft of the transmission, and have no losses through the torque converter.

I like that way Josh. I'll keep that in mind. Unfortunately, I don't know the gear ratio of 4th + lockup on my 4EAT :-p

 

MOST automatics are direct drive (1:1) in third gear. This is true of MOST manual trannies with either third gear or fourth gear, however every Subaru gear ratio chart I've ever read has no direct 1:1 gear, not sure about the older models though, haven't dug too deeply into anything 83 or so or older...

here are some numbers to play with from an austrailian subaru site. I can't imagine them being mush differnt then any other models

http://www.subaru.com.au/models/outback/2.5i/wagon/specificiations/

 

nipper

You guys are making it more difficult then it really is.

 

Take your engine rpm at 55 mph and divide by the gear ratio of the gear you're in.

 

Since you're probably in an overdriven gear, you will get higher rpm output (driveshaft rpm) then the engine rpm.

 

eg: 3000 engine rpm / .987 (5th gear, gear ratio) = 3039.51 rpm

 

That only takes into account the gear ratio of the gear you're in at the time. What about the final gear ratio ?

That only takes into account the gear ratio of the gear you're in at the time. What about the final gear ratio ?

 

They were only wanting to know the speed of the drive shaft.....drive shaft is before the final gear reduction.