speedometer too slow

[mickeypowa]

Hi guys !

I have a problem with my speedometer :
when I drive at for exemple 50 kmh, it shows 30
when I drive at 90 kmh, it shows 50
as you can see on the video :

also, the needle is not accurate, it goes up and down.
I tried to remove the speedo cable to put oil on it and I put it back but it changed nothing.

I know that I have not the original transmission on my car, it has been changed by this one :
https://www.summitracing.com/int/parts/hup-35-1/overview/

So maybe I should change the driven gear by another with less teeth on it ? How can I know what number of teeth I need ? With the rear end ratio ? How can I know it ?

I found this :
https://gbodyforum.com/attachments/20161009_202419-jpg.55387/

But I have a th350 transmission, so what is it ? a 6kcf, or 6bhb, or others ? etc ?

Thanks a lot

Mickeypowa

 

[MrSony]

Left hand drive Cutlass, cool man. Speedometer cable gear inside the transmission is probably broken, it's plastic so it breaks over time. What size tires and what rear end ratio do you have? That will determine what gear you need.

 

[Texas82GP]

Hi guys !

I have a problem with my speedometer :
when I drive at for exemple 50 kmh, it shows 30
when I drive at 90 kmh, it shows 50
as you can see on the video :

also, the needle is not accurate, it goes up and down.
I tried to remove the speedo cable to put oil on it and I put it back but it changed nothing.

I know that I have not the original transmission on my car, it has been changed by this one :
https://www.summitracing.com/int/parts/hup-35-1/overview/

So maybe I should change the driven gear by another with less teeth on it ? How can I know what number of teeth I need ? With the rear end ratio ? How can I know it ?

I found this :
https://gbodyforum.com/attachments/20161009_202419-jpg.55387/

But I have a th350 transmission, so what is it ? a 6kcf, or 6bhb, or others ? etc ?

Thanks a lot

Mickeypowa

For the money, this may be the way to go....
http://m.ebay.com/itm/18-19-20-21-2...LET-Powerglide-TH350-/131761841557?nav=SEARCH

 

[mickeypowa]

my speedometer cable didn't seem to be broken when I removed it totally from the tube. But maybe the part on the transmission is used, I have to check that.

I don't know my rear end ratio, I've seen videos on youtube of people puting tape on the driveshaft and on the tire and they count the number of turns of driveshaft for 1 tire revolution. Should I do that ?

 

[UNGN]

I don't know my rear end ratio, I've seen videos on youtube of people puting tape on the driveshaft and on the tire and they count the number of turns of driveshaft for 1 tire revolution. Should I do that ?

I've used the tape on the tire/driveshaft method many times to determine rear axle ratios in the junkyard.

Spin the drive shaft and make sure both tires are spinning the same amount. Any differential action will taint the results.

 

[MrSony]

You could also pop the trunk and look on the trunk lid for a white rectancle sticker with a bunch of letters and numbers on it. Those are the ROP codes that tell you every option your car has from color to wheels to rear end ratio. Take a good clear picture and we can determine that for you. It will look like this. If you can't find it in there, the tape on driveshaft method will work fine. The sticker should look like this:

 

[drogg1]

You could also pop the trunk and look on the trunk lid for a white rectancle sticker with a bunch of letters and numbers on it. Those are the ROP codes that tell you every option your car has from color to wheels to rear end ratio. Take a good clear picture and we can determine that for you. It will look like this. If you can't find it in there, the tape on driveshaft method will work fine. The sticker should look like this:

I believe his car is a 1980 and so it is too old for trunk stickers. I believe they started in 1984. In my 1983 I found the crusty build sheet under the rear seat which has (or had at some point) the different option codes. Some people have found them above the headliner or underneath the carpet.

 

[mickeypowa]

you are right, I don't have anything on the trunk lid. maybe because my car has been re-painted.
I have bought a jack today, I will try to know my rear end ratio tomorrow !
I will have a look to find this rop code plate maybe under my rear seats too ? 😉

 

[565bbchevy]

If none of the less expensive ideas work you could have a gear ratio box made that will get your speedometer back to reading right.
I had mine made locally but if you provided the the differences in MPH you could probably order one online.

 

[Texas82GP]

you are right, I don't have anything on the trunk lid. maybe because my car has been re-painted.
I have bought a jack today, I will try to know my rear end ratio tomorrow !
I will have a look to find this rop code plate maybe under my rear seats too ? 😉

Your car was built before the use of RPO stickers. RPO=Regular Production Order. There will not be a plate under the seats. What the others were talking about above is the "build sheet", which is a sheet of paper that lists all of the equipment to be installed on that particular car. It is a document used by the factory. Sometimes the build sheet was placed under the back seat or above the headliner or above the gas tank. You will be better off using the tape.