I am trying to figure out why I turn such high RPM's at low speeds. It was suggested to check my axle ratio. There is no tag on the rear end.

I jacked one wheel up, spun it 2 times exactly and the driveshaft only went around 3 3/4 times. Everything I have read says I need to divide that by 2 to get the ratio. That would make it 1.875:1. That can't be right. At that ratio I shoud be able to go 150 MPH!

I did it about 5 times and got the same number every time.

Thoughts?

Jeff

 

Read THIS. What's messing you up is that one wheel is still on the ground. I'd say you've got 3.73s, just like the example.


"This situation is exaggerated when you jack up only one rear wheel while leaving the other on the ground. One wheel cannot rotate at all, so the other wheel (the one you're turning) must make two full turns in order to get one full rotation of the ring gear in the differential. One rotation of the ring gear will generate 3.73 rotations of the pinion gear."

 

Excellent...makes sense. Thanks a lot for the info! Using the chart it looks like I'm about where I should be on the RPM's at 70.

 

This one is very helpful because it allows you to leave any value blank.
Gear Ratio Calculator.

 

It's definitely going into 3rd. My RPM's are probably closer to 4000 at 70. According to the chart 336.1355/ 25" (i have 205/70/14's) X 3.73 X 70= 3510 + 10% for no lockup= 3861. It is also an old converter, so I might have more slippage than the 10% I'm guessing.

 

I would throw a cheap tach on there to be sure. That seems really high for 70mph.

 

Yea I know. I've been trying to figure it out for a while. I was actually getting ready to throw in a different C4 and new converter tomorrow and see what happens.

 

i calculate 3700 rpm for a manual with your setup with no slip. those tyres and (3.73 based on the 1.875 you read). you seem to have open rear end. for that spin one wheel and double the turns of driveshaft.
so it seems to make sense all that

 

open diff. tape a string to the bottom of the driveshaft then push the car forward so the tires make one full revolution, count how many times the string warped around the driveshaft.

 

If your RPM is that high, you either need an overdrive transmission or to swap the gears for 3.00-3.20.