my '73 has a muncie trans, i believe to be original, and did have a 90 degree adaptor when i first got it, although the speedo was broken.
i since have had the speedo repaired, and found the adaptor to be defective.
with the speedo cable plugged directly into the tranny without the adaptor, my speedo reads increasing speed variance over actual speed.
currently, at 60mph i'm turning 2700rpm (does this mean i have the 3:55 diff?).
i can see the end of the 'speedometer drive gear' just at the end of the transmision 'fiting', and it is blue.

?do i need to change the drive gear?
which one?
should i replace the 90 degree adaptor?
any source for these.. i haven't found one yet?

 

are you sure it's a blue driven gear? i have the speedo gear chart for 73 here in front of me and there is no mention of a blue gear.

pull that gear out and count the teeth. how many does it have?

the right angle adapter on my 73 is a 1:1 ratio, so it should not matter if you bypas it and attach directly to the transmission. i think it just relieves stress on the cable. what exactly is wrong with your adapter? can it spin at all? can you check if the ratio is 1:1 for yours?

you will need to know what your rear end ratio is. there are two ways to find out.

1) jack up both rear tires. make a chalk mark on the axle and on one tire. count how many times the tire has to rotate in order for the axle to make one complete revolution.

2) crawl under and look at the under side of the differential. there will be some numbers and a two letter code.
AW = 3.08:1
AX = 3.36:1
AA = 3.55:1
AB = 3.70:1
AC = 4.11:1

you may want to do both of the above just to make sure. (i did.)

what size tires are you using? 255/60x15?

if your transmission is original to the car, your spedo drive gear will be an 8 tooth steel gear. your speedo driven gear is selected baised on your tire size and rear end ratio.

here is a site to help you determine the proper driven gear for your car: http://www.novaresource.org/speedo.htm

here is another site that even has a nifty little tire diameter calculator : http://www.tciauto.com/tech_info/speedo_gears.htm

the driven gears are available from places like ecklers and gmpartsdirect.com. not sure about the right angle adapter, but you could take a look there.

i'm going through this exact same thing on my 73. i think the car originally had a 3.08 rear end and a previous owner switched it out for a 3.55 and did not care that the speedo was off. my transmission has a pink 19 tooth gear in it which the book says is the combination for a 3.08 rear end. i ordered a silver 22 tooth gear because that is what the book says for my combination and i verified this using the web sites listed above. i also ordered a red 21 tooth gear because the calculations on those web pages said that is was very close as well. i'm going to try both gears and see which one is actually the closest to being correct.

good luck and i hope this helps.

mike

 

thanks for your reply mike! great info!!

yah, the 90 degree dosen't spin the cable at all, but i do have the cable going direct now anyway (might be nice to take some of the tension off though.. anyone have ideas where to get this??

funny thing to add, maybe you have an idea about this mike... ?

yesterday, on my way home, i noted that the spedo was actually registering the correct speed! then again this morning, it's back to registering 65 when going 60. (i checked my tach with mile markers and a stop watch to know true mph). WHAT"S UP WITH THAT!!!?

as far as checkin' the gear, and the diff ratio.. i guess i know what i'll be doing tonight!

 

Think you got it backwards, rotate the tire one time and count how many times the drive shaft rotates. 3= about 3.08 gear, 3.5 times = 3.55 3.75 = 3.70, 4 = 4.11

Tire will not rotate once with 1 turn of the drive shaft.

 

well, i'm pretty sure that when i did this about a week ago or so, the tire made 3.5 revolutions to get the drive shaft to go arround once. i could have it backwards, but this is the way is seem to remember it being. maybe somebody else can chime in and set the two of us stright.

if nobody else sets us stright, i will go back out , jack it up, and do it again. i don't want to be giving any body some disinformation plus i want to know if i am right or wrong. i could be wrong.

either way, i think H2O-V will get the idea when he goes to figure out what his ratio is.

 

thanx guys!
i haven't had the time yet as hoped, and am traveling next week, so it will have to wait a bit.

.. but more importantly..

i had the same experience again going home yesterday as mentioned before...

in the morning, with the tach on 2800rpm, which equates to 60mph, speedo shows 65.
at night going home, tach the same, speedo shows 60!

what the heck??!!

 

maybe you should grease your speedo cable while you are at it.....

just a thought.

 

I belive that 3.08 uses a white gear,3.55 blue?