is it possible to unbolt and remove the rear extension of a 1973 m20 muncie 4 speed without compleatly removing the transmission from the car?

my speedo reads about 10mph too high. when it says 50, i'm actually doing about 40. running throught the formulas i've found on diferent web sites is telling me that i need a 9 tooth drive gear and a 24 tooth driven gear. i'm running 255 60 15's with a 3.55 rear end, if anybody would like to check my math, i would appreciate it.

what would the standard drive gear size be for an m20 with 3.55 rear end from 73? the driven gear that came out was a 19 tooth.

i would really like to get this speedo reading right with out removing the transmission if possible.

any help or suggestions would be appreciated.

thanks,

Mike.

 

ttt

 

What formula's are you using? What websites?

 

i used this link to get the tire diameter:

http://www.tciauto.com/tech_info/speedo_gears.htm

255 60 15 = 27.05" tire.

this link was used to select gears:

http://www.novaresource.org/speedo.htm

rear end ratio * 20.2 / tire diameter = gear ratio

3.55 * 20.2 / 27.05 = 2.651

look up in the charts on the second web site to find closest gear ratio which is 9 drive and 24 driven.

i actually had another web site that i liked better because it used tire revolutions per mile instead of this magic 20.2 number, but i can't find the link and it gave me the same 2.651 ratio anyway.

if this is not the right way to do this, could somebody please point me in the right direction?

i would love to NOT have to monkey with the drive gear if it is possible.

 

A speedo shop can set up a speedo adaptors(or whatever they called these)if none of the plactic driven gears will work.
The factory 4.11 rear cars used them.

 

ttt

77 views and nobody has had this problem????

everybody with an early C3 just goes by the tach?

i don't believe that....

 

OK, car came originally with GR70x15 tires, which is pretty close to
225 70 X 15, which works out to 27.40 inches.

Using the formula in the second web site gives you 2.617...
which puts you in the 8 / 21 teeth category (originally).

However, you said you have a 19 tooth driven gear in your car.

I can believe that. The factory parts book has a chart for speedo
gears. It says (for 1974, which I pretty sure is the same as 1973,
but I will have to double check) that 3:56 rear end uses a 19 tooth
driven gear, WITH AN ADAPTER, part number 1565211.

Do you have the adapter specified? It probably attaches to the
transmission and then the speedo cable attaches to it. It would also
provide a 90 degree change in the direction of the cable, if memory
serves correctly.

Is the 3:55 ratio the original ratio for this car? If it came originally
with 3:08 ratio, it would have had the 19 tooth driven gear, but
would not have had an adapter.

 

i do have an adapter. it does provide a 90 degree bend. i will have to crawl back under the car to check on the numbers. does your parts book say what kind of reduction or increase the adapter applies to the cable speed?

i am not 100% certian that the rear end is original to the car. i don't think they put a partial vin on these, did they? all i know is that what is there now is 3.55

if the 225 70 x 15 is 27.40" and the 255 60 x 15 is 27.05" then this is probably contributing to error in my speedo.

how do i calculate the proper gears while including the effects of the adapter?

 

Tought to say unless you know the effects of the adapter. My book does not tell me the efffects of the adpater, just that it is there.

Many speedo shops can custom build you an adapter for situations when you change gear ratios, tire sizes, etc. Check Ebay for "speedometer adapter". I seem to recall at least one company advertising there.

 

ya know, i compleatly forgot about that speedo info in the assembly manual. i'm home now and can look at it.

the 73 is diferent from the 74 that emccomas was talking about. on the 73 the 3.08 rear end gets a 19 tooth gear and the 3.55 rear end gets a 22 tooth gear. neither gets an adapter.

i do have an adapter, but when i checked it this afternoon, the ratio was 1:1. it does nothing other than turn 90 degrees.

the transmission is original and had a 19 tooth gear in it. this leads me to believe that the rear end is not original. a PO must of changed it from 3.08 to 3.55.

i guess i'm going to order the 22 tooth gear and see if this fixes the problem.

thanks for the replies. they were very helpfull.

Mike.

EDIT : actually, when i do the math for my tires with the 3.55 rear end and an 8 tooth drive gear, i get a 21 tooth driven gear. since the PO didn't bother to change the driven gear, i'm positive that he did not change the drive gear which acording to the assembly manual is an 8 tooth gear. i'll probably get both the 21 and 22 and see which works better. they aren't that expensive.

thanks again,

MIke.

 

The tire size difference is so small I would ignore it. The ratio of the 2 rears 3.55/3.08 =1.15. 1.15 x 19(teeth) = 21.9 teeth.
I would say 22 teeth should do the job. Be carefull installing it since the center to center distance is now greater, the driven gear flange must be rotated (not sure how the Muncie is) to suit new larger gear. On the 700R4 the flange is an eccentric so you rotate it to get different center distances. Good luck.

 

thanks, mooneyd.

i already ordered both gears though, 21 and 22.

what you are saying about the diameter makes sence, but for some reason in the literature i have on it says that they are the same diameter, and the 19 tooth is the same as both of them.

i will have to measure the two new gears when they get here and compare them to the original 19 tooth.

it seems like some where in my searching on this topic it was said that the gears are cut in such a way that this diference in diameter does not effect installation. some have deeper teeth than others or something like that. i'll have to measure them and see whats going on...

thanks again,

Mike.