Hello, first post here, just bought a new to me 1994 convertible at Carlisle last weekend.
When driving I noticed the speedo seemed to be off. I checked it against my GPS and it is. At around 30 MPH the speedo is aprox 6-7 MPH low and getting to around 60 MPH the speedo is reading 50 MPH (10 mph off low). The lower speeds seem to be around a 5-7 MPH reading too low and as you go faster it is 10 MPH off. I have done a fair amount of searching on the forum and could not find a possible solution.

Further car information: Tires are correct size 285/40ZR17 in the rear and 255/45ZR17 in front. The car has the RPO codes of 1YY67(base convertible) G92(performance axle) and G44(3.07 ratio)

I do not see signs of any mods done to the car in the way of rear gears or anything. I am hoping this could just be a faulty VSS but thought it would be best to ask before chasing parts.

Thanks for any help, Mike.

 

Sounds like ratio change. Record MPH at 40, 50, 60, 70 on the GPS and check some measured miles and record. Yours should be an easy fix after confirmation.

 

I will take the car out later today and post all of my GPS readings Vs. the speedo. I am also going to put the car on a lift and spin the driveshaft and mark the wheel to get an idea of what gear is in the rear instead of going off of the RPO codes.

 

I did some mathematics for you.

There is a 3.54 'thick gear" set available - I know because I recently installed one on my own 1992 convertible.

IF a previous owner changed the gearing from the 3.07 to 3.54, and did not recalibrate the speedo, the true speed at an indicated 50 mph would be 57.5 mph. But, GM always programmed the speedometers to read LOW by 2 or 3 mph, for liability reasons in case anyone put on slightly larger tires, so I can see the speedo now reading 60.

If your car is a 1994, I THINK it may be correctable via the computer, versus via physical speedo gears.

I am asuming, though you have not said, your car is an automatic. If so, if it feels lively at all, it IS a 3.54 and not a 3.07.

Jim G

 

Oops! I got that backwards. If they changed to a numerically higher ratio, and did not correct the speedo, it would be reading higher than actual, not lower.

If they change from 3.07 to 2.59 for low rpm highway cruising, you would see what you are seeing. But then, the acceleration is the opposite of "lively".

Jim G

 

Yes, sorry to not add the car is an automatic, 4L60E.
The car does not seem to be a slug off the line, doesn't feel like 2.59's.
I do have access to a lift and will check the generic way by spinning the driveshaft and counting wheel revolutions later today.

Also I know the car is OBD 1 and most likely be fixed by the computer instead of the gears. I also have access to a snap on SOLUS scan tool that I will also scan the car for codes and I really don't know if the SOLUS can fix the problem if it is a programming problem with the computer.

 

I realized the car was a 4L60E from your original post! The options that you listed were enough to confirm that. Don't be concerned with anything other than recording the speedo error and the odometer error for the time being. If your scanner will scan for the VIN you would like to confirm that from the PCM. It was a "for sale" car so who knows what's actually going on. No rush to judgment!

The "old school" rotation check is done by one tire revolution equals how many of the pinion.

The "old school" way is going to be the way to confirm what's actually in the car but the GPS/MPH & odometer record will be used to confirm that. The tachometer isn't actually reliable enough to use for an accurate comparison.

If you have a VIN confirmation from the PCM a reasonable assumption would be that the tune & PCM maybe haven't been messed with. Regardless of what's found yours is a fairly easy "fix"!

 

Thanks, I will get those numbers in a little bit, hopefully sometime later today.

 

You might consider adding your "location" to your profile or just mention your location! Then after checking perhaps someone has experience with someone local to you that can correct the PCM for "what-ever"!

If the car runs well, seems to do well in the fuel consumption arena I would probably start with just the speedo/odo correction. Don't be tempted to get "carried away" just yet. You don't need to be "sold" a performance tune just yet!

**While you're under the car you might as well do a confirmation of something else. Check the rear differential cover and see if there is 3 bolts across the top or 2 on the top. You're looking for one top center at 12 o'clock.

 

I got out on the road and under the car.
here are the readings speedo v. GPS
Speedo reading GPS Reading
20mph-------------23mph
30mph-------------36mph
40mph-------------47mph
50mph-------------60mph
60mph-------------73mph
70mph-------------85mph
I also went under the car and spun the driveshaft and marked the tire. For one revolution of the tire, the driveshaft turned a little more than 2.5 times.
I checked the stamp on the diff. it reads as follows

605490-4
10163874
2.59 44-17
2mar1992

So it seems as if I have 2.59 gears in the rear. It must have been changed out sometime in its lifetime. So now that I have that info, how do I fix the speedo?

Also, I did look at the rear dif cover and there only appear to be two bolts on the top, I cannot see one at 12 oclock but it is tight under there and that is what I can see. Didn't get a chance to put the scan tool on it yet.

 

3.07 /2.59 = 1.1853xxxxx so if you do the speedo X 1.1853 you're reasonably close to the GPS values. I'd say question is answered. Now to get the PCM done it will either take a "tuner" or you'll learn and buy the equipment. Since it was a factory available option (2.59) in the days when the vehicle was considered a current product GM would assemble the flash for a dealer to correct tire size and axle ratio. I doubt with the age of the electronics that's possible.

I have a 3.07 D36 with fresh bearings, seals that has been on the shelf for a while. That would correct the speedo and offer up some SOP also. The sale of your 2.59 down the road would recover some of that. It sounds like you could likely handle the replacement your self.

I believe you'll find the bolt at 12 0'clock when you look. I was actually anticipating that the car had a gear installed like a 3.54 and the PCM was adjusted and then the rear was removed for the sale of the car and the results would have been similar to what you spec'd in the MPH/GPS.

In maybe the scan & tune or maybe even the general forum post a request for help and see if there's someone close by that can handle the calibration for you. I'd avoid the "mail order" - "Internet" buys until I looked close and considered what you believe are the fellows "skills"! Something to think about!

Ask the seller where the 3.07 is? Was the buy a "bargain" or would you say it was priced above market? Seller might offer to participate. Might be worth the ask. What can you get told? NO

 

I really didn't want to get too into Turning wrenches too quick, but putting in the 3.07 could be a good winter project. WV-ZR1, pm me and let me know what you want for the d36 3.07 rear, we could probably work something out.
In the meantime I just want to get all the "bugs" out of the car before I get into a big project, so I will post a request in the tune section and hopefully get someone around here to flash the PCM.