determining speedo gear needs

[bird_dog0347]

So I changed to 3.55's in the rear when I installed the IRS and my speedo is way off... it shows 60-65mph while I'm doing roughly 45 MPH (flow of traffic).

According to the calculator below the only thing I don't know for sure is the transmission year/drive teeth as my car was a 4 banger swap originally and I didn't source the motor/trans. Is there an easy way to tell how many teeth the drive gear has or what year the trans came from?

http://www.latemodelrestoration.com/products/Mustang-Speedometer-Gear-Calculator

[90MarkVII]

How many teeth on the current cable gear?

[90MarkVII]

Or...download one of the dashboard apps for your phone (assuming you have a smart phone) that will give you actual MPH. Note the difference, do the math and determine which tooth gear you need.

[homealone]

Pull the speedo cable and look inside. Unless you're color blind, 7 tooth yellow, 8 tooth green.

[bird_dog0347]

Pull the speedo cable and look inside. Unless you're color blind, 7 tooth yellow, 8 tooth green.

I'm not color blind, I'll do this, thanks!