Well, the first thing to determine is do you really have 3.31 gears in the rear as advertised. You can jack the rear wheels up, support the car with jackstands, put the car in neutral. Mark the driveshaft and then start turning. Have someone watch the rear tire and count the number of revolutions the driveshaft makes to make one complete turn of the rear tire. It sounds like someone changed the rear end and neglected the speedo gear.

alan

 

You need to verify the tac reading also. My 3:31 runs about 2700 rpm's at 60 mph.

 

Congrats on your new buy, enjoy it. You're one of the 'Wingers.'
As Alan suggests, check the driveshaft-to-wheel revolutions, the easiest test.
A highway measured mile is 60 MPH = one minute.
Try another GPS unit?
I have two stock equipped vehicles, meaning tire size per decal, and both with stock 3.73 rear gear, both turn 3000 RPM at 60 MPH by their installed tachometers.

After checking d/shaft-to-wheel ratio, I'd see if clutch is slipping - select 4th gear and floor it. If clutch is slipping, rpms will climb like no tomorrow. If clutch isn't slipping, acceleration will be quick because if 4250 rpm at 50 MPH is correct, you might have a 4.88 gear, or more.
As Steve suggests, is tach reading correctly? You could borrow an aftermarket tach just to check or visit a diesel shop and have them strobe it at 3000 rpm and compare readings between the strobe tach and installed tach.

 

Well, the problem is if they changed the ring and pinion without the right speedo drive gear, he has no idea what his speed is unless someone with a known good speedo paces him on the freeway at 60mph.

alan

 

If it's really doing 4000 RPM and your speedo says 52, then it's not the right speedo gear for 3.31's either.

 

True. If it's turning 4000rpm at 60mph, I'd think the gear is lower than 4.10s as well. If counting the revolutions of the drive shaft compared to revolutions of the tire doesn't seem to compute, pull the cover off the rear end and divide the number of teeth on the pinion gear into the # of teeth on the ring gear. That will give you the ratio.

 

If the car has the original trans and speedo pinions and rear gear was changed to a higher ratio, the speedo would indicate a faster MPH.

A PO may have tried to correct the reading and maybe couldn't get to where it should be.

 

That sounds more like 4.11 gears not 3.31's.

 

X2, But I would pull the cover , count teeth, Probably needs a gear lube change anyhow. Don't forget the posi additive.

Once you know for sure, get the driven gear to match.

 

Sounds more like 4.56

 

4.88. or 5.36 if the tach is right which I'm thinking not

Use this calculator.

http://www.csgnetwork.com/rearendgearcalc.html

The OP said he has GPS so he knows the actual speed.

With 3.31s and a 27" tire he should be doing 2500 at 60 mph

 

I did not see any mention of tire size. They don't always make a huge difference, but they are apart of the package for speed/rpm issues.

 

this may sound crazy but maybe he just needs to shift gears?

 

On a side note since I've got a 4.10 geared car that still has the 3.73 speedo gears in it....

Do you have to change out the gear in the tailshaft at any point or do you just have to know what gear is in the tailshaft so you get the correct tooth count speedo gear to prevent stripping due to incorrect mesh?

 

As I recall and it has been 20+years there are different colored speedo gears to match the rear gears and tire size but I don't remember having to change the transmison drive gear. I can't remember offhand where I found the info, maybe the ASM.

 

There were two different diameters of pressed-on steel 8-tooth drive gears on the output shaft; the "large" drive gear only worked with the "family" of small-diameter driven gears, and the "small" drive gear only worked with the "family" of large-diameter driven gears.

If the car originally had a 3.31 axle and now has been changed to a 4.11 or 4.56, you'll need to change the drive gear on the output shaft in order to use the correct "family" of driven gears to make the speedo read properly.

:beers:

 

Hey guys, FINALLY got to get the car out of the garage today and drive her for an hour or so, in order to form a relevant response to all of the very helpful posts. I really paid close attention to my gauges, It seems my memory was a little off. According to the car's instrumentation I'm at about 3100RPM at 50mph and 3600RPM at 60mph.

Again, I am sure the spedo is off by about 8-10 mph due to having someone follow me in a "new" car with accurate spedo, and checking with GPS. If I'm really going about 70mph then 3600rpm may not be too unreasonable.

This is in 4th gear, so unfortunately I am out of gears 68z28sd!

Everett#2390 - I've got plenty of torque in 4th, clutch is still strong! I've got 15" rally wheels with stock-sized rubber. Car originally came with 14's w/spinner hubcaps, but I would be surprised if the new set is too far off from the original overall diameter.

The car is going in for some suspension work and body bushing replacment (worried about rusted mounting plates, want it to be in a body shop or I'd do it myself!), so I will see if they can confirm engine RPM for me. That is my biggest concern for getting an accurate reading. I will also see if they can swap out some lube in the rear and count the teeth while doing so, to confirm the original 3.31 or a replacement.

John Z- I will certainly swap the 3.31 back into the car if the one in there is not stock. I'm keeping this car to original spec, considering all of the other surviving components and being a true RS/SS.

Thanks again for all the suggestions, I will respond when I find some answers. It may be a while, this is a long-term project!

Rick

 

OK, STOP! Welcome to Team Camaro.:beers: