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I recently had my 93 lt1 automatic 40th anniversary corvette's rear differential swapped due to damage. My RPO codes claimed I had a 2.59 ratio, but when I replaced it with another c4 2.59 ratio differential, my speedometer is showing that I am going slower than I actually am. I'm not sure if the previous owner had any work done to the rear differential. The mechanic that did this obviously didn't know what he was doing, and he didn't communicate anything. Is there anything else that needs to be done other than replacing the speedometer gear? Any help is greatly appreciated.
I recently had my 93 lt1 automatic 40th anniversary corvette's rear differential swapped due to damage. My RPO codes claimed I had a 2.59 ratio, but when I replaced it with another c4 2.59 ratio differential, my speedometer is showing that I am going slower than I actually am. I'm not sure if the previous owner had any work done to the rear differential. The mechanic that did this obviously didn't know what he was doing, and he didn't communicate anything. Is there anything else that needs to be done other than replacing the speedometer gear? Any help is greatly appreciated.
IF you had other than a 2.59 and now replaced with a 2.59 you would need gear and a correct VSS. This is assuming the speedo/odo was corect. Is the busted rear still at the shop? What did you expect the shop to communicate? Did he supply the rear or did you? If the old rear is still around you need to inspect it for content.
He inspected it and passed it. At 50 mph on the gauge I am definitely doing something closer to 60. Maybe even a little bit over 60. I don't have access to the old one. Any chance you have a ballpark on how much those two would cost?
He inspected it and passed it. At 50 mph on the gauge I am definitely doing something closer to 60. Maybe even a little bit over 60. I don't have access to the old one. Any chance you have a ballpark on how much those two would cost?
Where is the busted rear? Did you buy from the shop or you carried him 'what you bought'?
For starters you need to confirm what VSS is actually in the car now. In the air for snapshots might be best. No one knows what's actually been done. Tire/wheel sizes on the rear as it is currently?
Very dependent upon what you find less than $100.
Not that it matters now but why did you need a rear? Post a snapshot of the SPID on the console door. Does the VIN on the SPID match the driver door and the windshield plate?
If you didn't feel like changing the VSS gear there are a bunch of options that would allow you to modify the output pulse of the VSS to match what it's supposed to be. Dakota Digital makes one. Also you can use an Arduino to change the pulse to what it should be. This is nice for people who change gear ratio's or tire size often.
3.08 is the performance axle ratio, and is fairly common to find from parts sources. It is very likely that if the previous owner changed the ratio, a 3.08 ratio was used.
The difference between 2.59 and 3.08 is 15%.
The difference between 50MPH and 60MPH is 16.5%
It looks to me that your old ratio was 3.08, and that the VSS currently installed is for 3.08. You need the 2.59 VSS parts.
This is perhaps correct and what I actually assumed likely BUT he doesn't know how the PO might have corrected the odometer! Did the PO use an ERA or did he maybe use a correct VSS and driven gear? NOT KNOWN YET and who knows perhaps a simple 'dip-switch' tweak or parts!!! This guy needs to LOOK!!!
Originally Posted by Phobos84
If you didn't feel like changing the VSS gear there are a bunch of options that would allow you to modify the output pulse of the VSS to match what it's supposed to be. Dakota Digital makes one. Also you can use an Arduino to change the pulse to what it should be. This is nice for people who change gear ratio's or tire size often.
He doesn't know and he can't correct anything 'until' he looks and gets acquainted!!!
If this guy inspected his car now and found this VSS then it could be assumed that an ERA was used for correction because mechanical correction requires a different VSS 'to be done correctly'~ If he finds one w/black electrical connector and 10456092 then it could be assumed the correction was done with gears. If he finds something 'other than' these two then who knows without an inspection.
See you don't get it!!! You don't know that it's NOT already done that way. "DIP-SWITCH' tweak and it could be fixed!
No I get it. It doesn't matter what method was used to correct or not correct the VSS signal in the past. If he got a VSS controller he could set it to whatever he wanted.
Also if he had a rear end changed and now his speed is off on the speedo the likelihood that the mechanic doing the work changed the electronics to compensate for gearing are slim to none. Most trans shops won't do that kind of correction (unless it's changing the plastic gear). That falls on the owner.
no i get it. It doesn't matter what method was used to correct or not correct the vss signal in the past. If he got a vss controller he could set it to whatever he wanted.
Also if he had a rear end changed and now his speed is off on the speedo the likelihood that the mechanic doing the work changed the electronics to compensate for gearing are slim to none. Most trans shops won't do that kind of correction (unless it's changing the plastic gear). That falls on the owner.
YOU REALLY DON'T GET IT If this guy finds a 10456091 VSS it might be safe to assume that he would just remove the DEVICE you suggested he buy!
I didn't think of the possibility that there is already an electronic adaptation in place. If so, the wires that need to be intercepted are in the engine compartment, probably at the ECM, and the boxes are usually not weather resistant, so the box will be inside the car. I guess look for "wiring changes" to find the box.
I'd probably go with the speedo gear was changed. Easy enough to check if the exhaust doesn't make access too fussy. (I don't know if it does or not, Its been 25 years since I worked on an automatic C4.)
The 2.59 speedo gear is 35 teeth, orange color, PN 10456091
The 3.08 speedo gear is 42 teeth, green color, PN 10456092
Pop the VSS out of the trans. If the gear ISN'T orange with 35 teeth, go find one. That's what it needs to be. Or add a calibration box. (Changing the gear will be cheaper and a LOT less work).
If the gear IS orange with 35 teeth, find the aftermarket calibration box and remove it.
The OP should NOT even have to remove the VSS to inspect. I would NOT take it apart until he confirm just the appearance and info off of the VSS. I'd think the shop would offer up the lift for the required inspection!!
The image in #8 is all the OP needs to confirm for the visual. #'s and if there are no numbers just the connector color black/white should suffice.
The mechanic told me what rear to look for,(c4 dana 36 automatic 2.59 ratio) I found it and then he confirmed it. The old differential is gone. Can I reach the diff to take pictures using a jack or two or do I need a lift? The rear tires are 285/40 zr17
Last edited by msroll44; Feb 11, 2021 at 08:32 AM.
It would be much easier I believe with a lift and I believe the shop owner who did the work should be happy to accommodate you. I'd do a few measured miles, and with a GPS make note of speed on GPS when the dash is @50, 60, 70. At those speeds typical 'actual' should be 59+ (59.27), 71+(71.12), 82+(82.97). To determine the least expensive way to move forward you have to look.
If it was done with an ERA there would likely be a box stuffed under the dash. I'd think easier to do a check under the car vs start pulling interior parts to 'sneak a peak'. If it was done 'under-hood' with an ERA look near the ECM for a box like the link in #9 or maybe a box like this one. It costs nothing to 'lift the hood'!!!
From: Liliha Bakery stuffing my face with coco puffs!
I think the goal here is to return the speedo back to stock. x2 on verifying speed with GPS. Provided the gears in the pumpkin is stock/never been changed, the ring and pinion ratio is stamped on the bottom of the pumpkin. I'm thinkin not all of these were available options??? https://www.corvetteforum.com/forums...ml#post1303920
Rear Differential swap concerns
