Speedometer waaaay off...

[Twilight Fenrir]

Well, here's a somewhat interesting one... My '66 toronado's speedo is waaay off... I have to drive about 65 to actually go about 55mph. I understand I can get gear-reducing things to stick on my cable and get it accurate, but i'm curious how it got so far off.. I put on a set of 235 70 r15 tires on stock wheels... Isn't that the right size?

[69W34]

Twilight ... Do you know for sure the full mechanical history of the car?

[Twilight Fenrir]

The tabs on the car when I bought it were from '81... it's been sitting since then.

Beyond that, the only maintenince I know is from what I've worked on and seen. Little seems to have actually been done on the car mechanically. The original owner took time to repair the body, but never did the mechanicals. With the exception of tune-up parts, and a few missing components, everything appears to be as it was the day it rolled off the assembly line.

[xgecko]

You are experiencing the same problem I have now that I put 235-70 series on the car. It does seem that there is approximately a 10% or so error; it is almost certainly due to the slight size difference between the original 8.85-15 tires and the modern 70 series. A 75 aspect ratio with a 225 tire probably would have been closer, but you lose handling with the taller sidwall.

You can obtain a gear that will fix this. It will not be an add on, rather, it will replace the existing gear.

Summit Racing has gears:

http://www.summitracing.com/search/Part-Type/Speedometer-Gears/Transmission/Automatic/Transmission-Make/GM/Transmission-Type/TH425/?Ns=Rank%7cAsc

The Speedometer drive gear is found in the housing that the speedometer cable enters. You can calculate your proper gear size based on the error. Here is a handy calculator for this:

http://www.sccoa.com/faq/speedgr.html

That should provide you with the information you need to fix this.

[xgecko]

FYI near as I can tell we have an 18 tooth drive gear... this from my factory manual chart.

It looks like a 40 tooth gear is most likely the correct gear. I am assuming the standard 1000 rpm of the drive cable at 60 which seems to be the most common value out there.

[Twilight Fenrir]

FYI near as I can tell we have an 18 tooth drive gear... this from my factory manual chart.

It looks like a 40 tooth gear is most likely the correct gear. I am assuming the standard 1000 rpm of the drive cable at 60 which seems to be the most common value out there.

Alright... just coming back to this as spring is drawing nearer....

As best I can tell from combining the information on your site (which offers tire dimensions and converts to height), with the formula on Jegs (which offers the tire height to rotations per mile figure)

# Drive Teeth x Axle Ratio x Tire Rev. per Mile
1001 = # Driven Teeth

Use the following formula to calculate the Tire Rev. per Mile for the above formula.

Tire Rev. per Mile = 20168/Tire Diameter (inches)

With 235 70 R15's, We wind up with:

(18 x 3.21 for me, 3.07 for you x 736)/1001= 42.5 for me, and 40.67 for you. (66/67 and 68+ have different final drives)

TCI makes a 44 tooth, and a 39 tooth.

After some searching... it seems 39 tooth is probably about as close as you're going to come.

However, apparently, I might be able to buy a 42 tooth gear on eBay!
http://www.ebay.com/itm/700R4-TH400-Spe ... 0407353505


The headache here seems to be... that the up to 39 tooth has a different housing than the 40+... So I'll need a new housing as well... Which will tack on a hefty sum...
http://www.ebay.com/itm/TH400-Speedomet ... dd&vxp=mtr

[Twilight Fenrir]

Faaaantastic... Well, this is what I get for buying the parts first, and looking at the orriginal only as I go to swap it out... The speedo gear has this kind of housing, not the above:



However, I can confirm the original speedo gear is a 32 tooth gear. It's also actually metal, and not nylon. I don't think I can get a speedo gear to fit :S I'm either going to have to live with it, or swap out the drive gear as well as the driven...

[Twilight Fenrir]

Well... It doesn't look like I can get my speedometer gears setup with this transmission. The housing isn't big enough to fit the larger gears necessary... So, I'm going back to my original plan, and buying one of the little gear reduction units that just screws onto the transmission, then the cable goes in... I need a 24.7% reduction, I can get a 26.7% reduction here:

http://www.transmissioncenter.net/speed ... ____va.htm

2% is a rediculously small amount. So it's close enough for my needs.

I've read a review here that seems to indicate it's a good company to deal with, and a good part:

http://www.fullsizechevy.com/forum/gene ... apter.html

Hopefully this thread will save someone else $70 worth of useless parts

[1977 Cutly]

I think this thread just saved me $70 worth of useless parts. I have two Olds that both need speedo corrections. I've decided to use the add on units.