[13s1975.0]

so im taking apart my 91 mustangs drivetrain as we care building the engine in the process of taking the trans out right now and i see that the gear is redish color (i added a pic attached) im also adding a pic of what the speedometer is reading when im drving 60 mph. im hoping with these yall can tell me what gears are in the car or guess, while i have it all apart id like to get the speedo reading correctly, also its oem tire size as well. also the needle kinda dances a little bit when below 20 mph im thinking a new cable will fix that but i dont know what to do about the speedometer being off so bad

 

[vinnyc5566]

do you have a stock ring and pinion in it?

 

[Dwain]

You need to look through the hole in the trans and determine the color of the other gear the red gear meshes with. There is a yellow, black or a clearish green color. Also post your rear end gear. The combination of the gear in the trans and the gear in the rearend dictates which gear you use on your sender. IN some cases you have to change the gear in the trans in order to get the appropriate ratio for you rearend gear choice.

 

[13s1975.0]

ok i will know tomorrow forsure i believe its black, seeing the rpm im running at does that look like 3.73 to you

 

[13s1975.0]

you mean look into this hole in the trans? and can you change the gear on the inside as well or just the one i took the pic of

 

[Dwain]

you mean look into this hole in the trans? and can you change the gear on the inside as well or just the one i took the pic of

Yes, use a flashlight and figure out what color it is. The one inside the trans can be changed too but it requires the tailshaft to be removed which isnt very difficult. Changing the one in the transmission is only needed if you can get get the correct ratios using the existing gear inside the trans which usually isnt an issue unless you have short gears.

 

[russ3747]

I would bet they are 3.73.

https://www.cjponyparts.com/resources/engine-rpm-calculator

 

[1991&135.0]

Ok this is me this is my current account I don't know why I was logged into the old one when I posted this tread anyway here is a picture inside, it's yellow so that would make it a 7 right? That with the 21 tooth would be correct for 3.73? I'm not sure if i have 3.73 but my speedometer has always been off about 10 mph like in the pic any ideas on how I can go about fixing that

 

[1991&135.0]

So if the speedo is correctly geared for 3.73s and the speedo indicates 70 when I'm actually running 60 would that mean I have taller or shorter than 3.73 gears or is something else the problem

 

[vinnyc5566]

My car has a t5 with a 3.73 gears, 275/40/17 tires and i found the white 23 tooth gear from lmr to me the most accurate in my set up. The LMR calculator on Lmr told me I needed red 21 and that was incorrect giving me an issue just like yours.

 

[1991&135.0]

good to know see i have no idea what gears are actually in the rear end im thinking i need to open up the diff and actually count and judging by the rpm im thinking i have 3.73. does your car also have the yellow gear on the inside

 

[Dwain]

just put the car in the air and rotate the tire 1 full revolution and count the number of times your drive shaft turns. That will tell you your gear ratio in the rearend.

 

[1991&135.0]

just put the car in the air and rotate the tire 1 full revolution and count the number of times your drive shaft turns. That will tell you your gear ratio in the rearend.

i have the engine trans and driveshaft out of it rn im guessing i can turn it and mark the yoke where the driveshaft connects

 

[Dwain]

i have the engine trans and driveshaft out of it rn im guessing i can turn it and mark the yoke where the driveshaft connects

Correct.