CentralCoaster

The answer to all your pesky speedometer gear questions:

http://webpages.charter.net/khasting...peedocalc.html


Let me know what you think or if you find any bugs.

Update:

I made a minor change, turns out EVERY SINGLE speedometer calculator on the internet is wrong. Mine was too until I fixed it just now. These other speedo calcs are based on tire diameter, not rollout. Rollout is more accurate because it factors in the tire squish from the car's weight. This depends on the tire profile. So I calculated it for every tire size ever used on the C4 and also checked tire manufacturer data for common vette size tires. They were very consistent with about 3.2% of squish of the tire's diameter.

The calculator is now dead nuts accurate, and if the stock GM gears differ from it, I can say with confidence it's because they screwed up. Basically its because they avoided using 41 tooth gears for some reason, even though it was optimal, they picked 40 or 42.

Also, that commonly posted speedometer chart is chock-full of errors. Specifically the speedo gears for 94 are wrong, and the speedo sensors for 91 are wrong, and some of the tire sizes, and who knows what else.

hippy

This is actually something I have a question about. Why do you need a new/different VSS is you have the proper gears?

CentralCoaster

There are different VSSs for each range of gears. The smaller gears need to be positioned closer to the transmission output shaft than the larger gears, so the VSS housing is different. The VSS internals are all the same and unrelated to your gear ratio.

I think the speedos can also just be installed upside down to change the offset. Worked for me anyways.

toptechx6

Nice Central Coaster, wish I'd had this before struggling to get mine right! It came up with exactly the correct results based on my trial and error method unlike other calculators I've seen. I like the tire size option rather than height since there seems to be controversy on the correct way to measure that.
I am still puzzled about the speed sensors, I changed mine to the correct PN 10456092 but even after very careful examination I could see NO dimensional difference compared to the original PN 10456091. In fact if there is an offset from center it is nearly imperceptible on either sensor. I ran my car briefly with the PN 10456091 sensor in the reversed position and it seemed to work fine.
Casethecorvetteman tells me none of this is necessary on 1994 and newer cars since they can be programmed to correct for the gear ratio. Casey also indicated those cars use a different speed sensor with no gear at all along the lines of a wheel speed sensor, yet this calculator shows 94's using the same gears as 93?
Can the newer cars be corrected using either gears or by programming?

CentralCoaster

On the link it says "not for 94-96 autos." But I will probably add something into the program that detects that input and gives a useful answer for those guys.

The 6092 and 6089 sensor are both "big gear" sensors so the offset is away from tht output shaft. They appear to be interchangeable.

The 6091 is a "small gear" sensor that offsets toward the output shaft, but can be rotated 180 to change the offset to handle big speedo gears.

I personally think that all the 84-90 sensors are interchangeable and all the 91-96 sensors are interchangeable depending on how you position them in the trans, but I'm not ready to carve that in stone. The calculator works anyhow.

In short, YES you may be able to avoid buying the correct speedo sensor and reuse your old one.

Steel Breeze

Hey Kevin, Not highjacking this thread, but I could tell you some stories about that rear end setup we were conversing about with PM's. By the way... You didn't get back with me...

CentralCoaster

I honestly have no clue what you're talking about. Do you mean the Ujoints and bearings?

Waiting...

Steel Breeze

Yes, That would be correct...

CentralCoaster

I sent you a big long one yesterday I remember. I cleared out my inbox since then, it was full, maybe you sent it then.

I would gladly accept donations toward paying for supporting member status though to get extra PM capacity.

Steel Breeze

You should have that as honorary status. Actually, I think I will post my findings on this forum w/pics, as I think all the members should have a look.

I'll get it done asap as I am coming down with something. I have NO immune system. Anything can be lethal, "seriously".

I don't want to clutter up your thread anymore Kevin. I will be in touch. I do genuinely appreciate your help...

wingstlr

Now that you posted that link i now get three different gear call outs. Bowtie overdrive says 41 tooth, your link says 42 tooth, and the link im found on the fourm says 43 tooth. i have a 93 auto with 275/40/17 and 3.54 gears.http://www.corvetteforum.net/c4/vett...Gearshtm.shtml

toptechx6

For what it is worth wingstlr the other 2 were incorrect for my application, same car as yours, same tires but 3.73 gears. After some trial and error with mine I am satisfied with the accuracy and the numbers match Central Coasters link.

wingstlr

I also talked to gm parts house they said the 43 was for me and so did pete k.

toptechx6

I bought several drive gears and tried different ones until the Vette speed matched my other cars. I have a 44 tooth in mine with 3.73's, stands to reason yours should need slightly fewer teeth.

Desert1957

Hey Coaster,
I thought I was done with this question, but here goes. "James93LT1" gave me this link to correct my speedo.I have a 92 ZF 6 Speed w/4.10's
Your link calls for a different combination of gears? whats up? who is right?


http://www.zr1specialist.com/HAT%20W...on%20Gears.htm

Desert

CentralCoaster

Hey everyone, I made a minor change, so if you used it before this post, recalculate it. Details explained in the first post.

CentralCoaster

BTW, your gear "specialist" appears to be charging $120 for a set of plastic gears.

These gears cost less than $10 each from your GM dealership.

_twisted_

Dude , you need a G/F
thank for posting the info

CentralCoaster

Only took about 20 min to make the changes. But yeah, I'm done here. I'm gonna finish up on the yardwork.

C4Techie

That link to the "ZR1 Specialist" lists gears and tooth counts that GM does not supply. I believe they are made from metal, and I have no clue as to their origin. If memory serves me, they are only necessary for the 4.09-4.11 gears. To check if any of the economical plastic gears can be subbed for the those exorbitantly priced gears, figure and compare the gear ratio of the speedo gears.