Hello Guys, Something that hasn't concerned me but maybe its time to correct. after the drive train was rebuilt 2 yrs ago, Trans and motor (Im sure most of you remember). I noticed at some point that the speed was off (reading) The speed is reading on the speedometer about 15 to 20% higher. I always see plastic gears on ebay (i believe color coded), is that what I need to correct? or might there be another reason this is happening?
If I need to change a gear, can I do that myself? or need to have the tranny shop do it?
Thanks bill v
69 Firebird Convertible (wifes car since 1979) Goldenrod Yellow, 350, plain Jane Car was stored in garage since 1990
I need help, if anyone see's I'm going down the wrong road--Straighten me out!
The speedo's in our cars were never very accurate, but you can try a new drive gear. You can try a new gear, Drive is in the transmission, driven is at the cable.
This will help you calculate what you need - You probably won't get the exact gear, but you'll get close.
As far as being able to do it your self, that depends on your abilitys. The driven gear that is the one in the housing that the speedo cable screws onto, it is easy, just remove the bolt that holds the housing into the trans, the drive gear, that is the one that is on the output shaft of the trans, you will need to remove the driveshaft and tailshaft of the trans to get to it. Also depending on the trans you have, the gears are different, and on the TH-400 there are different speedo gear housings for the different range of gear teeth count. Here is another source to determine the gears you need for the speedo, it is from a Nova Site that I am on also, but will work for GM cars.
Measure your actual tire height with a straight edge of some sort, round up higher if it's an off number because the tire will be higher when driving, don't go by the tire manufacturer claims as they are always wrong and you wheel width will change the height, scout around because some places sell the driven gears for $15 and some $5 for the same product.
You need to know the tire rpm, revolutions per mile, to find that you divide the tire height by 20168
ex 20168/26" = 775
then multiply the drive teeth (found by tranny, most muncies are 8) X axle ratio X tire rpm
Thanks guys, I'll have to get under there and take a look to see if I can figure out what I'm dealing with. If i can get that gear out easily I goof around with some of those calculations you guys are describing and see what I come up with. Worst case scenario, I have a friend that has a tranny shop, in fact he rebuilt the tranny for us as we were waking this bird up after 2 1/2 decades. I believe the tranny is a TH400, its one of the few non original parts on the car, It may have come out of a Cadilac , maybe a 76. Thanks again, I will keep you guys posted
Bill V
69 Firebird Convertible (wifes car since 1979) Goldenrod Yellow, 350, plain Jane Car was stored in garage since 1990
I need help, if anyone see's I'm going down the wrong road--Straighten me out!
I remember seeing a gadget that you could order that would go between the trans and the speedo cable. It would change the ratio. You use your phone gps to measure your actual speed to determine how far the speedo is off at 60 mph. Then they custom build a unit for you.