Discovered a bonus with my recent purchase

[Paul Workman]

Originally Posted by WARP TEN

Great discovery. If you do have it is the best mod for your car to improve performance noticeably without doing major work. You can also simply "count" the axle with the car in neutral, handbrake off, one rear wheel on the ground so it doesn't spin, one rear wheel jacked up. Make a mark on the driveshaft you can see, a matching one on the diff housing, and one on the wheel. The drive shaft should turn 4.1 times for each wheel revolution. If it only turn 3.4 times, stock ratio.

One leetle problem dar Bob! Fergittin 'bout the positraction? Gonna be tough rotating the driveshaft with one tire on the ground! Best both wheels off the ground and counting the driveshaft rotations per one wheel rotation.

I

[Ccmano]

Originally Posted by WVZR-1

OP - I assumed maybe the same when I first got my '90 but I had been told it was original! The '90 tachs are known to be very inaccurate and I discovered it was stock after using the ATC panel to calculate RPM and I connected a Tech1 to confirm that I actually had a 3.45 and the cluster needed attention.

When you turn the KEY TO RUN with the car "not running" if the tach needle isn't parked on 0 you have a tach error. Needs to park on 0 - not close - on 0!

Use the ATC if it's a C68 car and calculate RPM using it for confirmations.

Good point and as I wrote that was assuming the tach was correct. I'll be checking that next. I have to say that based on my perception of engine rpms I'm inclined to think it has the 4:10's but we shall see.
H

[BigJohn]

I have some 5:13's!

[efnfast]

Big John, is that what they run in the Callaway? 5.13's? Wow.

[XfireZ51]

Hans,

If u have GPS, u can use that to confirm.

[WARP TEN]

Originally Posted by Paul Workman

One leetle problem dar Bob! Fergittin 'bout the positraction? Gonna be tough rotating the driveshaft with one tire on the ground! Best both wheels off the ground and counting the driveshaft rotations per one wheel rotation.

I

Good point, Paul. As I recall from my (very) old days summer job as a track mechanic at Ford's Dearborn test track, it was not a problem but I don't recall truly if we dealt with posi axles back then. I do know that with open rears we had to have an assistant hold the other wheel while we counted the axles. So if you are right that both need to rotate, then jack up both wheels and let them rotate together. That should work I would think. --Bob

[WVZR-1]

Originally Posted by XfireZ51

Hans,

If u have GPS, u can use that to confirm.

GPS can only be used to confirm Speed & odometer - I still don't get the u vs you - LAZY?

This isn't a TEXT exchange - it's a "conversation"!

[BigJohn]

Originally Posted by efnfast

Big John, is that what they run in the Callaway? 5.13's? Wow.

I don't think they would fit in a corvette!
The 5:13's are in my plow truck.

[Ccmano]

Originally Posted by Ccmano

Good point and as I wrote that was assuming the tach was correct. I'll be checking that next. I have to say that based on my perception of engine rpms I'm inclined to think it has the 4:10's but we shall see.
H

Had a chance to take her out today, with the laptop attached and Datamaster running.

Bob, you were right, tach is off considerably. The speedo is correct. At 60mph the correct rpm is about 2800. Based on the formula that is right for a 3.54 diff ratio.

Also noticed that's the tach is really off at high rpm. When I shift at 7+k on the tach I'm barely at 6k in reality.

Anybody have any thoughts on how to get the tach fixed?
H

[WVZR-1]

Originally Posted by Ccmano

Had a chance to take her out today, with the laptop attached and Datamaster running.

Bob, you were right, tach is off considerably. The speedo is correct. At 60mph the correct rpm is about 2800. Based on the formula that is right for a 3.54 diff ratio.

Also noticed that's the tach is really off at high rpm. When I shift at 7+k on the tach I'm barely at 6k in reality.

Anybody have any thoughts on how to get the tach fixed?
H

KEY ON - engine not running the needle isn't parked at 0 either. That's the quickest "first check"!

There are a couple possible ways to correct the tach. In the past few years there's very likely a much improved procedure than what was used for years before.

Mine was actually done by an GM/ACD cluster facility that I had a relationship with at the time. My entire cluster and display was confirmed. The tach correction has been a DIY for many but much depends upon the persons talent.

I don't know how expensive a "for hire" today might be but if the "for hire" included a check/correction of the entire cluster that could be a very good way to proceed.

I sent a circuit board to PW many years ago for him to sort out a project he had at the time.