ok well i had a shop install 3.73's and a posi into my car. i had a set of speedo gears so they said they would put those in also. when i went to pick up my car they said that i got the wrong speedo gears. i got 15/43 gears. my info is 255-50-16 with 3.73's

 

Giving the trade size of your tires is worthless as an exact indicator of tire size.

Go measure the ACTUAL RADIUS of a rear tire, at normal pressure with the car sitting normally on it; from the exact center of the wheel to the ground.

According to my calculations however, with tires having a radius of 13.125" (diameter 26.25"), the 15 & 43 combo will spin the cable approximately 998.96 rotations per mile. 1000 / mile is ideal. Therefore, your speedo should read approx .1% lower than actual speed; i.e., you're going 100 mph, your speedo will read 99.896 mph, give or take a couple of inches per hour.

Something tells me, that's close enough.

Now as to whether those speedo gears are the "right" ones, as compared to what the factory used, I have no clue. There are other speedo gear combinations that come out pretty close to what you'd need for that rear end & tire combo, that the factory could have used instead; which call for a speedo gear ratio near 2.864:1.

But if you can tolerate an error of about one-tenth of one percent, then what you've got should be OK.

 

sorry i must have used the wrong wording. i know that 15/43 is the right numbers. what i was asking is that the shop told me they are not physicaly able to put them in. they said i got the gears for a mechanical speedo and i need one for an electronic speedo, or vise versa, not sure which one they said. but it is a 91 700r4 trans. sorry for the confusion

 

Well that's a whole different matter.

If this is a 91 car, it should have the electronic stuff. The gears your link goes to are for cable drives.

Go here http://www.transmissioncenter.net/sp...n_______va.htm and scroll down about 1/4 or 1/3 of the page.

Depending on what gear you USED TO HAVE, you might have to replace the sensor as well. I'd say that's pretty likely, since you probably had 2.73 gears before.

 

i will be going from 19/40 to 15/43, will i need a new sensor?

 

It wouldnt hurt to change it while you are in there. As for the gears I think those #'s are the correct ones. If the drive gear is red and the driven is green that should be correct then. hth