Speedo is off but the formula says it should be spot on. I'm at a loss

[84 C20]

I've got a 1984 C20, Th350, 9.5" 14 bolt with 3.73 gear. In the transmission is a 9 tooth drive gear and a 21 tooth driven gear. The stock 235/85/16 tires complete the numbers.
The old speedo was off around 5 mph then I broke it..... The brand new speedo says 30 mph but the gps says 47 mph. Instead of breaking this one, I've decided to do some research and the numbers say the speedo should be reading my correct speed. As far as I can tell, no one else has had this weird problem. I'm starting to think the speedo is reading incorrectly. It's not bobbing/jumping and the cable turns smooth as butter.

Any thoughts? Any things I could be overlooking? Maybe I'll just velcro my phone to the gauge lens...

 

[84 C20]

Called a few transmission shops and they said that all the internals of TH350's are all the same (in terms of gearing, not quality) so that isn't the problem.

on the plus side, I can honestly say 'But officer, I was only doing 50.'

 

[Georgeb]

What are you using for a calculator?

 

[Georgeb]

Try this thread.......
http://www.gmsquarebody.com/forum/showthread.php?t=1252

 

[84 C20]

That is the thread I read to get the calculator to figure out what gears to buy. When I put them in the original speedo read about 5 mph slow. So, in my infinite wisdom, I elected to try cleaning out the unit. All I accomplished was breaking the needle off. That's what prompted the new speedo and this confusing issue.
Over the weekend I did notice the needle bobbing sometimes. Is it possible the break in the cable is clean enough as to exhibit this type of behavior? That's my new theory. Based on nothing but guessing.... I am planning on using most of the upcoming holiday to pull the gauge package apart to fix the speedo, fuel level; really all of the gauges don't work.

 

[87scotty]

I'm happy with give or take 5mph as long as its not enough to be ticketed for lol I drive my 87 like an old timer though

 

[PrairieDrifter]

I would check the ends of the speedo cable to see if they are wore down. I want to say that the connection at the transmission is plastic but im not positive about that..

 

[Georgeb]

In the past I have had to play mix and match with different gears until I got it right. It helps I have a bag full of different gears to work with. I lucked out and got a bunch from the dealer I worked for and also grabbed them whenever I saw one in the salvage yard. I can't seem to remember how many mph change happened for each added or subtracted tooth but there was info on it somewhere.

 

[84 C20]

Thanks for the input. Here's some more info I found out:

I talked to the Classic Auto Parts people and they report they've never heard of such a thing and I should talk to a transmission shop.
The transmission shops tell me there's only one set (numerically) of gears that get put into a TH350. There are different quality of gears but the number of teeth don't change. So that blew one of my theories out of the water. I was hoping there was a different gearing set in the transmission which would make all of the math off.
I've had to get new tires over the holiday and what a difference that made! The speedo is still off but the streets have gotten sooooo much smoother! Plus, they are much grippier. I'm fondly remembering the days when the speedo was off only 5 mph!

So, I need to get a bag of plastic gears and play around huh? Oh joy. I loooove dropping the drive shaft... I'm getting good at it though.

 

[SlickGTP]

I like using this site... you can play around with the drive gear numbers to figure out the different combinations that will work for your set up.

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

[84 C20]

I've used that TCI page; like it quite a bit!
Update: I changed out the speedo cable and put in an 18 tooth driven gear. Apparently I've got a bind in the cable somewhere because the needle is bobbing a bit. With the lower tooth gear the speedo is still a bit off; the numbers tell me a 17 tooth gear will get me very close to accurate. So I ordered one from American Hot Rod today.
There's a page I found this morning (http://www.bgsoflex.com/speedo1.html) that allows you to play with the speedometers rotations per mile. According to the information on that page, most of the speedo's are calibrated for 1000 rotations per mile. I found that by changing that number to 1200, all the original numbers in this thread work out: 9 and 21. So I think it's the speedo that is off but I'm hoping the 17 tooth driven gear sorts it all out.
Thanks for all the input! I'll update with results of the new driven gear. Then my next project is to get the fuel gauge to work.

Also, I poured a bit of transmission fluid into my armpit when I pulled the bullet out. Another learning lesson.... ha!

 

[84 C20]

Fixed.


I fixed it by pulling the speedo I got from Classic Industries and replacing it with one from LMC.

the guy from classic says they've never had one that was defective. I believe that to be a load of crap but that's my opinion. they said send it back to them, they'll test it and maybe return a vague amount of $ at some point in the future. So, Rock Auto and LMC will be getting my money during the reno of my truck.