Guys,

My speedometer reads lower than my actual speed by about 5mph and I have been wondering if I have a different rear gear than what was originally in the car (GU6 3.42).

Well, I jacked the rear of the car up and when I rotate the wheel I am only getting between 1.5 and 2 revolutions of the wheel to one revolution of the driveshaft. So something isn't right!

I did this with the car in neutral, which I believe is correct, because with car in gear the wheels spin, but not the driveshaft. I did notice there is a fair amount of gear slop.

So, what am I doing wrong or what is wrong with my rear end?

 

Thought I should add that I do not believe the posi is worn out because the last time a I did a burnout (couple months ago) I had two good black marks.

I really don't think it has a very high gear either because it will keep the tires spinning through 2nd and into 3rd!

 

You turn the wheel one revolution and count the driveshaft turns. Not the other way around. Both rear wheels in the air.

 

Ok, well since it is a ratio, that means one revolution of the wheel gives me less than one revolution of the driveshaft.

How can that be...

 

The other tire has to be turning backwards or you're counting revolutions wrong. My posi is junk but in a straight line my car will spin both tires 99% of the time. Jack it up again and watch the other tire as you turn the one and see what happens.

 

When I turn one tire forward, the other does rotate backward.

I am not a retard about these things, not that you were saying I was, which is why this is so confusing to me. This is supposed to be a simple straight forward thing.

 

When i put different size tires on my car it changed my speedo and made me about 5 MPH off. This is a possibility. What size tires and amount of tread left?

 

It can't be, if things are like they should. I have done this many times and it always works. Perhaps your posi clutches are not holding. Have you tried using the other rear wheel.

 

If your other tire was rotating backwards then your posi is junk and you'll never be able to get an acurate reading because your tire is turning instead of your driveshaft. You'll want to get a friend to help you by turning the other tire at the same speed you're turning the one, then count the driveshaft revolutions. I'm guessing your wheels and tires are slightly taller/shorter than stock and are the cause of the inaccurate speedometer readings.

 

Pull the cover and check the internals for the gear ratio & limited slip. You can waste your time turning the wheels .

 

Damn! That is what I was afraid of! This should be an easy straight forward process, so I know something must not be right.

Well, if the posi is bad, what are my options? I was thinking about doing gears in the future anyhow, should I just do a posi unit in addition or is the cost so great at that point that I should just get a new rear all together?