What is the correct speedo gear for 2.73?

[Foxnotch28]

I figured I would come here to ask since there is a discrepancy amongst the speedo gear calculators I have found. I was told my '91 Mustang was switched to 2.73 in the rear when I purchased it. Speedo is not working right at all and I know I need to change the speedo gear because I passed one of our roadside police speed displays and I am off by more than 10 mph by what was showing on there. Someone told me I should get the yellow 18 tooth gear because that came stock with 2.73 Mustangs. Others have said I need the white 17 tooth gear. I have not pulled the current gear out to see what is in there. Just wondering if anybody else has had experience with this.
My car has:
2.73
215/60/16 tires
8 tooth drive gear (90-95 aod and aode)

 

[ALMOST STOCK]

I'm coming up with a Gray Gear

Automatic Transmissions with 8 Tooth Drive Gear



1990-1998 Automatic Transmissions



Item Number
Description
C7AZ-17271-B
2.73 Gear Ratio / Gray 18 Speedo Gear

 

[Foxnotch28]

ALMOST STOCK, I appreciate your input. Which calculator did you use? Maybe I should pull the gear currently in the tranny and see what's in there. The latemodelrestoration calculator says basically a 17-tooth gear. I just want to get this thing fixed and not buy several different speedo gears in the process....but I think that's where this is headed.

 

[ALMOST STOCK]

I didn't use a online calculator instead I used the Ford Racing site and did the math.

http://www.fordracingparts.com/download/charts/142.pdf


At the very bottom of the page it give you the formula for figuring the correct gear, in your case it would be
8X 2.73 X 815 = 17.799…. rounded off to 18 = Gray

 

[Foxnotch28]

ALMOST STOCK, thanks for helping me out again. Think I will go with that gear and see what happens.

 

[ALMOST STOCK]

No problem, like everyone else here just trying to help if I can.

 

[Foxnotch28]

I got the gray 18 tooth gear and swapped it in. There was a 19 tooth gear in there. Won't know for sure if the speedo is accurate until I pass one of those stationary speed indicators the police leave around town, but I'm pretty sure it's reading correctly. May have to replace the speedo cable though. It didn't turn too easy when I was trying to spin it by hand. I sprayed some NAPA cable and chain lubricant in there. Definitely working better than it did before the lubing. It was jumping around like crazy when you sped up or slowed down before.

 

[Foxnotch28]

I don't think the gray 18 tooth gear is correct. Passed 2 speed indicators today and I would say the speedo is reading between 5-10 mph slower than my actual speed. I need to replace the speedo cable too. The one from Late Model Restoration seems like a good deal, but one of the reviewers said it won't work with a speed sensor. Any advice?

 

[ALMOST STOCK]

Curiosity....... have you checked to make sure that you do have 2.73 gears?

 

[Foxnotch28]

I have not checked that out. I'm going to buy a replacement speedo cable and speed sensor because I am really tired of the needle jumping around during acceleration and deceleration. It's possible this could be making for the incorrect reading as well. Just my luck, Late Model Restoration is out of stock on the cable right now. Anyways, I think I should replace the cable and sensor before deeming the speedo gear wrong.

I have a used speedo cable from another car, but I am apprehensive to use anything other than a new cable. Don't want to lube up a used one to throw in there. No telling how long it would last. Most places I've looked say the speed sensor is the same for manual or automatic transmissions. The one at LMR -an oem Ford piece- says manual trans only. I just want to get this all sorted out and not have to spend more money than I need to.

 

[ALMOST STOCK]

I understand exactly what you're saying about solving one problem at a time and not spending anymore money than you have to.

 

[Blown347]

You can get in the ballpark of what rear gear you have by putting a mark on one of the rear tires or drums and rotating the driveshaft while its in neutral and watching the revolutions of the axles for every revolution of the driveshaft.