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Old 11-18-2013   #1
Pumpman
 
Join Date: Nov 2013
Location: Cape Coral, FL
Posts: 20
Default Speedometer calibration

I just bought this 91 and I found the speedometer reads about 10 MPH higher than my actual speed. While it helps to avoid speeding tickets it is annoying.
Can anyone advise me if this can be corrected and how?
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Old 11-18-2013   #2
ZZZZZR1
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Default Re: Speedometer calibration

Hey there

Need more info:

Does your car have a different rear gear? 4:10's?

Any other modifications? (Changed for, 4:10 back to stock?)

Speed sensor replaced recently?

Do you have 17' tires?

Here is the important question... New owner??????

If so, welcome!!!!!!



David
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Old 11-18-2013   #3
Mystic ZR-1
 
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Default Re: Speedometer calibration

Dave's never going as fast as he thinks he is!!!
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Old 11-19-2013   #4
WVZR-1
 
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Default Re: Speedometer calibration

Quote:
Originally Posted by Pumpman View Post
I just bought this 91 and I found the speedometer reads about 10 MPH higher than my actual speed. While it helps to avoid speeding tickets it is annoying.
Can anyone advise me if this can be corrected and how?
Sometimes is very easily corrected - other times not so easily. If you posted just the tire size and the speedometer reading when GPS is 40 - 50 - 60 - 70 MPH the percentage of error could be calculated. Your speedometer can't be off 10 MPH at all of those speeds. We're looking for a %.

If it's had an axle ratio change and nothing was done to the VSS gear or the drive/driven as a package then it can be corrected. How easily? Yet to be determined!!
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Old 11-20-2013   #5
Pumpman
 
Join Date: Nov 2013
Location: Cape Coral, FL
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Default Re: Speedometer calibration

Yes, I am new to the forum and just purchased this car in October. In 1991 I didn't have $60K laying around for this Vette I always wanted.
The tires are indeed 17" as far as the gear ratio is concerned I considered that as a possibility but don't want to tear out the rear end to count teeth. I checked the speedometer at 40 MPH mine read 50 MPH also at 50 MPH my speedometer read 60 MPH those were the two points (common in city posted speeds). As far as any ratio I cant say. Short of a gear change I am curios if there is an electronic correction that can be made.
I also own a 73 C3 but put a Doug-Nash 4+3 in it many years ago. That was an easy fix as the speedometer was cable driven and my local shop built me an inline gear box to correct.
Appreciate any help.
I went to a local speed shop with this speedometer question and the owner dissed the car said it was junk as one needs to buy a spare for parts. He also said rear ends gear sets are unavailable for these cars.
Really disappointing for a guy pumped up about his new ride.
Aside from the speedometer issue I have some light oil residue on the underside and around the bottom of the engine I need locate the source of and correct.
I have never enjoyed driving any car as much as this one. My 73 is a handfull in compasison.
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Old 11-20-2013   #6
DaveK
 
Join Date: Jul 2004
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Default Re: Speedometer calibration

As other people have noted it could be one of several things.

1. Rear end gear change (4.10s or similar)
2. Problem with the Vehicle Speed Sensor (VSS)
3. Problem with the instrument cluster

On these cars the speed is calculated electronically; no mechanical connection to change.

I would start off by contacting the previous owner to find out if the rear end ratio was changed. If that's the case it can be corrected in a couple of ways.

a) Get a different VSS gear (plastic gear that drives the electronic pulses). This is a fairly easy fix, unfasten the vss cable, remove the stock plastic gear and replace with the new. I had this issue when I bought mine. I got the correct gear from Marc Haibeck (green for 4.10s if I remember correctly.) www.zr1specialist.com.

b) Get an electronic signal convertor. These can change the number of pulses coming from the VSS to whatever you need. All the ones I could see when I was looking required splicing into the electronics and seemed like they wouldn't last very well under the car so I went with the gear.

For either of these options you'll need access to the underside of the car.

If it's the VSS itself that's going then again it's relatively simple. Unhook the old one, drop in the new. The VSS is a little hard to find (or was when I did mine). I got one from Kurt White (www.whiteracingproducts.com).

The instrument clusters can be rebuilt, but I've never done that and have no contact info. I'm sure someone else will post up on that option.

Whatever you do, don't let people put you off enjoying the ZR-1. I've had mine for seven years now and other than very minor issues it has been extremely reliable, even though I put several thousands of KM on the car each year including typically one long road trip - I've driven to Bowling Green for the ZR-1 Gathering several times as an example which is about 3000km (1800mi) round trip .

I heard all the "negatives" when I was looking for mine including comments from people in my local corvette club and ignored them. The Registry is a great place to share information and learn how to look after the ZR-1 and these cars are virtually bullet proof.

Light oil on the underside is not too uncommon. The oil pan bolts have a tendency to work a little loose over time - something I also encountered. Tighten them back up and you'll probably be fine.

All the best and enjoy your new baby!

Dave
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Old 11-20-2013   #7
WVZR-1
 
Join Date: Oct 2004
Posts: 1,884
Default Re: Speedometer calibration

Quote:
Originally Posted by Pumpman View Post
Yes, I am new to the forum and just purchased this car in October. In 1991 I didn't have $60K laying around for this Vette I always wanted.
The tires are indeed 17" as far as the gear ratio is concerned I considered that as a possibility but don't want to tear out the rear end to count teeth. I checked the speedometer at 40 MPH mine read 50 MPH also at 50 MPH my speedometer read 60 MPH those were the two points (common in city posted speeds). As far as any ratio I cant say. Short of a gear change I am curios if there is an electronic correction that can be made.
I also own a 73 C3 but put a Doug-Nash 4+3 in it many years ago. That was an easy fix as the speedometer was cable driven and my local shop built me an inline gear box to correct.
Appreciate any help.
I went to a local speed shop with this speedometer question and the owner dissed the car said it was junk as one needs to buy a spare for parts. He also said rear ends gear sets are unavailable for these cars.
Really disappointing for a guy pumped up about his new ride.
Aside from the speedometer issue I have some light oil residue on the underside and around the bottom of the engine I need locate the source of and correct.
I have never enjoyed driving any car as much as this one. My 73 is a handfull in compasison.
I don't believe you'll ever regret the purchase of your ZR-1 ! EVER!

The oil pan bolts are frequently loosened in time and that's an easily accomplished fix generally by just retightening the hardware. I don't recall the torque specs and there a just a few that you can't easily access. Of course there could be other issues but none major.

The speedometer error can't be 10 MPH in error at 40 MPH and then 10 MPH again at 50 MPH to display 60 MPH. I've done speedometer calibrations for years and the cable driven "inline box" is something I'm very familiar with. I built many. An "inline box" as you described is actually a "ratio adapter" and is assembled to either raise or lower the ratio. The more practical and desired approach with a VSS is to correct the mechanical ratio before going to an "electronic box" to correct the error.

Here's what I surmise has been done - the rear axle ratio was changed to a 4.10 and the accepted/desired "correction" is to replace both the drive and driven gears when the ratio is changed. After the fact it's an effort to accomplish. I've seldom ever suggested the ERA but yours might be the "first".

"DaveK" - was apparently responded while I did some math and I can only mention that YES THERE IS A MECHANICAL CORRECTION and that is actually what he's referring to and maybe doesn't realize it. The mechanical change is from 15/42 (drive/driven) to 13/43 (drive/driven) to accomplish the correction. You will NOT require a VSS to accomplish the mechanical fix.


The correction using mechanical correction requires the purchase of $125 parts I believe and the drive gear (in the transmission) requires the removal of the drive-shaft and C-beam to gain access to the transmission.

The error on the speedometer for what I assume has been done is 47 MPH at 40 displayed and 59 MPH at 50 displayed.

Which box - I'll rethink those and comment. Maybe others have used the a box. I'd really suggest the mechanical correction but I understand if there's issues with the expense.

You could learn to drive at these #'s


Indicated speed Actual speed



10 --------- 8.4
20 --------- 16.8
30 ---------- 25.2
40 --------- 33.7
50 ---------- 42.1
60 --------- 50.5
70 --------- 58.9
80 --------- 67.3
90 --------- 75.7
100 --------- 84.1

If you leave the "city limits" try a drive at 60 MPH (GPS) that should display maybe 71 -72 and a drive at 70 MPH (GPS) that should display maybe 83 - 84. Those two additional speeds I believe would confirm my thoughts - no phone calls!

Last edited by WVZR-1; 11-20-2013 at 11:43 AM.
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Old 11-20-2013   #8
Pumpman
 
Join Date: Nov 2013
Location: Cape Coral, FL
Posts: 20
Default Re: Speedometer calibration

Gentlemen,
Thanks for the detailed replies. I will investigate and get the right gears for the speedometer pulse driver.
Will the VIN# or other numbers on door jam/console cover give me the code for the original rear axle ratio?
When I bought this 1991 car I assumed it was 390HP. I had read someplace that the upgrade in heads to 405HP did not occur until 92 but the data plate on the cup holder cover says LT5 405HP, 385 Ft-lbs torque, 11:1 compression.
Is this correct?
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Old 11-20-2013   #9
Mystic ZR-1
 
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Default Re: Speedometer calibration

"I went to a local speed shop with this speedometer question and the owner dissed the car said it was junk as one needs to buy a spare for parts. He also said rear ends gear sets are unavailable for these cars"


Your speed shop guy has his head up his a##!!!
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Old 11-20-2013   #10
batchman
 
Join Date: Jun 2009
Location: I live at Devens, one run at a time
Posts: 454
Default Re: Speedometer calibration

As far as I know all ZR-1s came with 3.45 ratio. The power band of the engine is very friendly to a quicker gear - a very desirable mod if it's in place, and no cheap to do right.

That tag appears to be a common "mod". Our car has one too, but it's not factory.

And don't let the opinion of one uninformed guy bother you one bit!

Cheers, and welcome,
- Jeff
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