jnsbill

Hard Question - my speedometer seems to be 10MPH slower than actual - checked the VSS with a scanner they matched speeds - because its constant not percentage that rules out speedometer gear?
so where do I look next?
Easy Question - I have floppy door panels, I've already made templates for reinforcing the ribs but I want to use the screw through the door panel into the early C4 tab -
whats everybody using for a screw that looks professional?
Dumb Question - I probably have to remove the halo trim to replace a rear targa mounting bracket and I don't know how the clothes hook comes off - in my 84 there was a screw that I could get to - does this one "Pop-Off"
THX

gerardvg

Hi.

What year C4?

there is a small plastic gear on the vss sensor that is on the driver side of the car, it can be removed with the trans in car.
Just unscrew the small retainer bolt, then wiggle the vss sensor out.
If the gear comes off just fish it out with a small screwdriver.

I have a turbo700 trans with 3.07 diff gears, 275 40 17 tires all round and my speedo is spot on with my GPS. (My VSS gear is a green one)Or 1800/1900rpm at 62 mph. So check your build sheet or RPO label in glove box or rear cargo storage area for early C4's, for the diff ratio.

Or raise the rear and spin the tail shaft one turn and measure how many times the wheels spin, one turn of the tail shaft may show 2.57 or 3.07 ie: 2.57 turns of the wheel per one turn of the tail shaft means you have 2.57 rear gears.

There are several vss gears to compensate for diff ratio changes.

There are also ratio adapters available, however they are quite expensive.

Some reading material below.

https://www.corvetteforum.com/forums...ter-gears.html

http://www.partshp.com/speedometer_gears.htm

http://www.corvetteforum.net/c4/vett...Gearshtm.shtml

jnsbill

sorry its a 95 - I will check those things today but wouldn't the wrong gear give me a percentage error rather than a constant?
THX

Nobody wants to answer my other question?

Raidmagic

For the screw through the door panel hit up a place like ACE hardware. They sell a nice screw with a beveled washer on it that would look good. On the coat hanger, it's been years since I had my interior apart. I'll look at mine tonight. I know as soon as I see it I'll know how it works.

jnsbill

thx appreciate your help

antfarmer2

Is your door panel loose on the back top of the pad?

WVZR-1

If this car is an A4 the speedometer calibration is in the PCM, the VSS is an electronic pick up with no drive/driven and actually I believe also for the M6 but there's still a mechanical drive/driven used for the M6. Have you done anything to the PCM like is it a used one that doesn't belong to the car? Has some one programmed/tuned it? Are you using any "plug-in device? Is the tire size or axle ratio changed? I believe the PCM used a digital ratio conversion so it could be a constant. I've never checked or intentionally screwed up a DRAC to check.

Which other question? The coat hook or the screw?

If the screws are the ones through the bottom of the door most anything with a finish washer should do well BUT you'll need inserts for the door. GM# 1377030 I believe should do that for you and you can find them nearly everywhere.

jnsbill

wow missed a lot today - A4 RPO says 3.07 gear - didnt get a chance to do much today except go for a quick ride with a GPS --- it is a percentage error - tires are 285/40-17 which measure approx 25 1/2 tall - I just got the car and the previous owner wasnt/isnt very knowledgeable about cars (he says) so I dont know if the PCM has been changed or altered - so I am looking more at the gear now?

@antfarmer - yes both door panels pop off the top rear when you close the door

@WVZR1 - the coat hook? on the P/S panel on the halo trim and thx for the part number (those are the white plastic inserts) correct?

thank you all

antfarmer2

My 94 did the same thing I just drilled a small hole at a slight angle and put about a two inch screw just under the window sweep. It hooks on the door frame. Put one front and back. It works great and can't see it at all.

jnsbill

theres one there already maybe its not long enough

antfarmer2

Or to much angle or to far forward.

WVZR-1

I wasn't going to debate the claim to a "constant" but I've certainly never seen a constant. Something has changed with the car, the PCM could be used and has the wrong calibration, a replacement PCM and it was calibrated wrong, a tune of the OE done in error OR the rear gear has been changed. There is nothing you can do with drive/driven gears to correct since it's a '95 and if the transmission is actually the E-build.

What is the percentage?

Does the VIN on the SPID label match the one on the door and also in the windshield?

I would do the rotate of the tire/wheel one complete rotation and count the pinion rotation to confirm the axle ratio first before any other attempt.

jnsbill

just cause I'm not the sharpest tack in the box - I wont be able to get the car on a lift to do the rotation thing so I am jumping ahead - "the PCM could be used and has the wrong calibration, a replacement PCM and it was calibrated wrong, a tune of the OE done in error" how do I kknow and how do I fix? and what is a SPID and where do I find it?

THX for putting up with a dumb guy

I will be able to do the percentage error in a couple of hours - will post

WVZR-1

SPID - Service Parts Identification (Service Parts Information Label) and I'd say that's where you got the 3.07 axle ratio information. In '95 it was moved to the rear storage area but some have mentioned still on console door so where ever you might find.

First thing is to check the ratio and worry about nothing until confirmed. Check the drivers door for the Tire Placard and I'd say your sizing is correct for nearly all of production. A Z07 car would have 275 X 4 but it wouldn't account for the speedometer error you mention.

On the SPID a 3.07 would be G44* (a correction) and if it were anything else it would be a 3 digit starting with a G. Not knowing where you got the 3.07 information it might be an interesting check.

jnsbill

WVZR-1 Its a 15% difference using a GPS - never new that the sticker was called the SPID - both VIN's match and I have RPO Codes G44 + G92
might be able to jack it up tomorrow - THX

WVZR-1

I corrected a 3.07 RPO code to G44, I was looking at an RPO list I don't use often and HE3 is still in that list. Oh well!

Now you mentioned it was 15% but you don't mention high or low so.

Mention your speedometer MPH and the GPS MPH so they can be compared. Ideally the way to do it would be do the speedo at say 35, 45, 55 & 65 making notes of the GPS documented at those speeds.

jnsbill

GPS Speedo
27 23 85%
37 32 86%
47 40 85%
57 49 86%
68 58 85%

WVZR-1

do the ratio confirmation but it would appear that a potential swap of the 3.07 for a 2.59 maybe and combined with maybe a calibration rounding and your rounding it comes theoretically close. One rotation of the tire/wheel and the count of the pinion revolutions is the what it takes here.

If that's not what's happened here it's likely a calibration error in the PCM from being either used, tuned or whatever. Does the PCM have any of the typical JY "yellow tire crayon markings" maybe a label from a rebuilder? If a rebuilder did the PCM and just picked a ratio that was maybe the more popular for the year and didn't bother checking the VIN. Post the last eight of the VIN and mention whether its a coupe or a 'vert.

jnsbill

should have time to jack it up tomorrow S5104915

jnsbill

finally got time - they are 3.07's but I listed the speeds backwards the speedo is 15% slower than the GPS - and now to show you who you are dealing with - the ECM would be that black box above the battery and the master cyl? if yes theres no strange markings on it that I see only a sticker that says 16181333 and BSE31238 - so where do I go next? BTW while it was on the lift noticed the VSS was leaking so we replaced it