3.55s Or 3.73s?

[Benz510]

Hey Stangnet,

Got a question someone will probably be able to answer. I'm in the process of figuring out what the PO of my foxbody installed on the rear end and i might just have figured it out. I know for a fact they are not the stock 3.08s that came on the car. I looked at a chart for determining what rear end gears i may have. I just replaced my speedometer cable so the speedometer actually works now. The problem is i also need to change the speedometer gear in the tranny because the speedo is off, but i need to know the differential ratio obviously.

I used a GPS and at about 2300 rpms it says I'm going about 65 mph in 5th gear on the freeway. I think they're 3.73s but what do u guys think?

 

[cleanLX]

need to factor in tire size as well.
use this site http://www.grimmjeeper.com/gears.html to dial it in

 

[A5literMan]

Most likely 3.73.

 

[Boosted92LX]

Jack up the rear of the car, put it in neutral. Put a chalk mark on the inside of the tire, and on the driveshaft. Roll the tire 1 revolution, count driveshaft turns. 3 1/2 turns = 3.55. 3 3/4 turns = 3.73. This will tell you for sure.

 

[cleanLX]

another thing to consider is simply doing the math to see how far off you are as a ratio...
i offer this because, it may not be possible to tell what drive gear is installed in the trans.
I think pre 88 T5's had a 7 tooth, post 88 had an 8 tooth... autos may be different...
So, if you pull the cable and check what color gear you have, and know what % your speedo is reading "off", multiply the speedo gear tooth count by the same % and bingo... you have the speedo gear tooth count required regardless of what trans drive gear or R&P gear you have.

 

[FoxMustangLvr]

Jack up the rear of the car, put it in neutral. Put a chalk mark on the inside of the tire, and on the driveshaft. Roll the tire 1 revolution, count driveshaft turns. 3 1/2 turns = 3.55. 3 3/4 turns = 3.73. This will tell you for sure.

@Benz510
Do what Allen (boosted92lx) suggested. This is absolutely the quickest and easiest way to find out what your rear gearing is. Then look up the calculator that @cleanLX suggested. It's hard to imagine that you have the correct speedo gear in your trans if you have yet to verify your rear pinion ratio unless the GPS gave you what you needed already.

 

[jrichker]

LMR to the rescue...

to figure what speedometer gear you need - eliminates guesswork; includes all the variables like tire size.
https://lmr.com/products/Mustang-Sp...2HBaMwOReEmr4w8jRMbXio1XyQ5BfVNf5AaAkVf8P8HAQ


Now that you know what you need for the correct gear, see
https://lmr.com/products/Mustang-Transmission-Speed-Correction

 

[Benz510]

Thanks everyone. I will try boosted's method tomorrow after getting my new exhaust setup inatalled.

I appreciate it. U guys always find time to help.

 

[FastDriver]

Stay away from trying to calculate based on MPH and RPM unless you have an accurate tachometer. The stock tach can be off by a lot more than the ~5% difference between those gears.

 

[jrichker]

Stay away from trying to calculate based on MPH and RPM unless you have an accurate tachometer. The stock tach can be off by a lot more than the ~5% difference between those gears.

There is a narrow sweet spot for the stock tach and it is moveable by calibration. However, outside that sweet spot, there can be up to 200 RPM off. I have the electronic tools to do the calibration, and I saw firsthand what you can and can't do with an 87-91 Mustang tach. The 92-93 models supposedly have a second adjustment trimpot, but since I haven't seen one, I can't verify that.

 

[FastDriver]

I defer to your expertise on the subject, J.