Speedometer Still Reads Same After Gear Change

[Ludigdrums]

Hey all...so, I've been have speedo problems for some time now, and come to finally see that it was the drive gear. A couple of teeth here and there were missing, causing my speedo to flop around quite a bit. Turns out, my blue drive gear is a 10 tooth (although I had thought it was an 8 tooth). So, with my new wheels and tires, and also not knowing the prior history of my car...I wanted to set the speedo straight. Using the calculators on the net, I figured that I need an 8 tooth drive gear and a 17 tooth driven gear.

I finally found a tranny shop nice enough to look through their parts and find me the "discontinued" from the dealer drive gear that I needed. He ended up just giving me a 10 tooth and 8 tooth.

Point of the story is, the speedo still reads 10 mph too slow (which I can't see how is possible). The one good thing is that the speedo is now steady. Although, it still moves a little bit when you hit a bump, or when you get into the much faster speeds like 85-90 and up.

I should also mention that it is a 165 MPH guage. I know for my '81 the stock only went up to 85 MPH, so this is obviously a replacement/upgrade.

Should I be assuming now that it is the speedo itself. I know that the gears are all good, the cables are all good, the transducer is around a year old and works fine.

Like I said, even if the speedo itself is screwed up, I can't see how it wouldn't be relative to gear changes ya know? Very strange!

By the way, my tires are 285/40/18 which gives a diameter of 26.98"

Thanks for any suggestions in advance!

- Jeremy

 

[Ludigdrums]

Come on guys....no thoughts?

Thanks!

- Jeremy

 

[Bob Chadwick]

This was a common topic a couple of years ago. Thank 77-4Speed, a member who is long gone but not forgotten.

 

[Ludigdrums]

Thanks Bob. Unfortunately, there wasn't anything there I haven't visited yet.

In the meantime, I replaced the driven gear with a 21 tooth, and it still reads the same as the 20. This makes no sense to me. The speedo cables are fine.

My lower cable has a bare end that goes to the driven gear and on the other end has a plastic tip that goes into the transducer. The upper cable has a bare end that goes to the transducer and I don't know what's in the speedo, but I think it's bare too. Is this the correct configuration?

The thing I'm thinking it might be is the transducer. Although, it's not that old at all. The reason I suspect this is because the cruise works, but when the speedo needle bounces, the cruise control reacts by either surging to keep up or letting down.

Any ideas?

Thanks!

- Jeremy

 

[Ken Anderson]

It sounds like your head unit. Whoever put the face on it did not recalibrate it and IT is reading incorrectly. I would pull the head and have it checked out.

 

[Bob Chadwick]

Wish I could be of more help but I haven't had any problems in this area and was just sharing what I have read.

The cables are square on each end and go into square configutions on each end. That included the speedo as well. I don't know which of the two plugs takes the plastic end.

Are you using a 8 or 10 tooth drive gear. It appears, though it is not 100% certain, that Russ' post was based on a 8 tooth drive gear. I don't know how to figure out which way it would go (up or down), but a 10 tooth gear would probably result in a different mph change than the 5 mph indicated with the 8 tooth gear. If a smaller change, you may not even notice it with some needle bounce.

Aren't you going the wrong way. Adding gears lowers the indicated speed on the gauge. Maybe drop down something in the 15 range, which is fairly drastic and see if it makes a difference.

Try PMing Dave L. as he seemed fairly knowledgable on this subject based on his reply to the other thread.

Bob