Speedometer calibration

[Pumpman]

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?

[ZZZZZR1]

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!!!!!!

:cheers:

David

[Mystic ZR-1]

Dave's never going as fast as he thinks he is!!!
:dancing :) :-D

[WVZR-1]

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!!

[Pumpman]

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.

[DaveK]

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

[WVZR-1]

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!

[Pumpman]

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?

[Mystic ZR-1]

"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##!!!

[batchman]

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

[WVZR-1]

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?

The "cup-holder" data plate is an "owner added item" - your console label should display a GM3 RPO code. The SPID label is located inside the console lid on the '91. That's the original 3.45 -If you wanted to lift both rear wheels and rotate a wheel one complete revolution if my thoughts are correct then the drive-shaft would rotate 4 revolutions +. If it rotates only three and one/half times then I'm lost. The advertised HP changes occurred in '93. Earlier advertised is 375.



Your speed shop guy has his head up his a##!!!

I'd have to agree with this comment!!!!

[Pumpman]

Thanks, I suspected the plaque was an owner "add-on". I will mark the DS and roll the tires to check the gear ratio.
And YES the Speed Shop guy was out of line. I will not patronize him.
My Dad worked as a mechanic 7 years in the Ford garage 1947 to 1954 changing parts. Then 7 years in the Chevrolet garage where he learned how to bench strip components and repair the broken parts. In 61 he went into business for himself and I worked with him.
One thing he admonished is NEVER - ever disparage a person's car even if it is a clunker (probably all they could afford).
I guess the Speed Shop owner didn't have a Dad like mine.
By the way my Dad later gave up the family business and worked in a Marina where he went several times to......Mercury Marine for training on outboards an IO's.

[WVZR-1]

One thing he admonished is NEVER - ever disparage a person's car even if it is a clunker (probably all they could afford).


There's many that could use that information "reminded" to them nearly daily. They just don't "get it"! I had to remind a couple probably more frequently than just "once a day"! Aggravating for sure!

[alnukem]

Where's the most reasonable place to buy the correction gears? Thanks.

[WVZR-1]

Where's the most reasonable place to buy the correction gears? Thanks.


The internal drive gear in 13 tooth that's required for any rear axle ratio over 3.73 is Marc - there's no need to shop it or even consider others. Just buy from Marc. I'd say buy the driven from Marc also. There's no reason NOT to. One stop shopping!

Marc's product page:

http://www.zr1specialist.com/HAT%20Web/products/Speedometer%20Correction%20Gears.htm

You do need to understand that these tooth counts for the driven are based on the 315/35 tire that's stock. The move to any of the 18" or 19" tire/wheel combinations require likely different driven gears so there's math and "loaded rolling tire radius or circumference" required. The advertised diameters don't work.

[WVZR-1]

I will mark the DS and roll the tires to check the gear ratio.


I don't know that this would make it any easier to calculate but if we changed the tire rotation to minutes you should see your "mark" every 14.6+ minutes or just less than every 90* rotation of the tire for the 4.10 ratio, if the original 3.45 were present every 17.3 minutes or 104* rotation of the tire. There's possibilities maybe of 3.73 every 16 minutes or 90*+ and a 3.91 which would be 15.3 minutes or 90* + very little. There's of course 4.33's or 4.56 possible but very unlikely.

Really large numerical ratios or medium duty trucks this has helped in the past. It's been a long time.

For this you could just place a piece of tape or a paint mark on the pinion flange deflector that would be easily visible. Most every build should have the sheet-metal deflector BUT not all.

[WARP TEN]

WVZR-1 is right--just count the axle. If you turn one rear wheel through one complete rotation the drive shaft will turn about 3 and 1/2 times for a 3.45, about 4 times for a 4.10 and roughly 3 and 3/4 times if it is a 3.73 axle. Marc Haibeck has the gears if find you have a non-stock rear and he can offer some very important hints to ease the job. --Bob

[csavaglio]

Another tip is to turn the wheels 10 times.....it makes the difference between 3.45, 3.73, and 4.10 a lot more apparent when you're looking for 34.5 turns, 37.3 turns, and 41 turns respectively. Sometimes the difference between 3.45 and 3.73 turns when you're looking at the shaft turn is tough.

I would suspect 4.10 would give a bigger difference than what the poster's seeing....3.73 would be more likely, but I haven't done a gear swap in a few years.

Chris

[WVZR-1]

Another tip is to turn the wheels 10 times.....it makes the difference between 3.45, 3.73, and 4.10 a lot more apparent when you're looking for 34.5 turns, 37.3 turns, and 41 turns respectively. Sometimes the difference between 3.45 and 3.73 turns when you're looking at the shaft turn is tough.

I would suspect 4.10 would give a bigger difference than what the poster's seeing....3.73 would be more likely, but I haven't done a gear swap in a few years.

Chris

If you do the procedure that I broke down in minutes it's a "single" rotation process. Done maybe "twice" to confirm but I've never had an issue where 10 rotations would have helped a thing. Of course you need to understand that I know some souls that can't count that high and are short some fingers and toes.

I did the numbers that I posted using advertised diameters because they should certainly be effective enough for the OP to determine his ratio IF in fact that is his issue. It may very well be another issue for the OP but it seemed the more rational explanation of what likely is going on.

The dash display should not have displayed the error in 10 MPH increments that he mentioned. He should have had a number that would have responded better with the math involved.

The numbers that I posted for comparisons should easily be interpreted by a drive of maybe 5 miles or so.

[Pumpman]

Gents,
Problem resolved. Car definately has a 4:10 rear-end gear ratio. Got the right gear set on it's way from Haibeck Automotive. $130 + shipping.
355+- degree turn of the wheel = exactly 4 turns of the driveshaft.
Haibeck is a great resource.
Thanks again for you help.

[batchman]

Cost of the gears is the tip of the iceberg. I'll bet you could find a C4 owner with 3.45 gear willing to pay you to swap pumpkins, especially if yours are quiet.

The only other thing to look out for is the condition of the limited slip. If you open yours to change gears you may as well rebuild the LSD. You want the Dana 44 HD kit from ring and pinion dot com.

[csavaglio]

If you do the procedure that I broke down in minutes it's a "single" rotation process. Done maybe "twice" to confirm but I've never had an issue where 10 rotations would have helped a thing. Of course you need to understand that I know some souls that can't count that high and are short some fingers and toes.

True that....... Sometimes it does make it easier when you're splitting hairs on gear ratios....the difference between 3.45 and 3.73 is only a quarter turn....the 10x method makes it easier to spot. Between 4.10 and 3.45, there should be no mistaking it either way.

Glad you got it figured out. Marc's a great guy to deal with. The ZR1 guys are lucky to have a lot of really good people to deal with.

Chris

[WVZR-1]

Cost of the gears is the tip of the iceberg. I'll bet you could find a C4 owner with 3.45 gear willing to pay you to swap pumpkins, especially if yours are quiet.

The only other thing to look out for is the condition of the limited slip. If you open yours to change gears you may as well rebuild the LSD. You want the Dana 44 HD kit from ring and pinion dot com.

The gears the OP bought are for speedometer correction, the drive and driven for the transmission and the VSS. He's going to enjoy the 4.10 - I doubt he'd ever want to go back!

[batchman]

The gears the OP bought are for speedometer correction, the drive and driven for the transmission and the VSS. He's going to enjoy the 4.10 - I doubt he'd ever want to go back!

Doh! Of course. The cost was pretty close to the last Ford 8.8 R&P I bought though - threw me off.

I'd be enjoying 4:10s if my class rules allowed. Rock on!
- Jeff

[Pumpman]

The 4:10 rear end is just fine. A little slippery on wet roads. Don't need to go back to stock just fix the speedo. Bought the correct speedo drive gear set from Mark and put them in on Friday. Not too tough of a job although dropping the exhaust system is the only way to go. Had to clearance a seam a tiny bit to gain clearance to pull the drive shaft other than that a piece of cake. Did it easily w/o a second set of hands.
I did use a black marker pen and colored the side of the output shaft gear just above the key slot. This made it possible to locate the slot and align with the spring key in the output shaft. Snapped right into place easily once aligned.
Speedo is right on the money now.
I have fixed all the little problems and time to enjoy.

[WVZR-1]

Good to see all "went well" and as expected!