speedometer calibration

[windriven]

I have just added 31s to my 91 4x4 ex with the manual 5 speed. I have found that the speedo is 10 MPH off. when going 65 I am acutualy going 72. My odometer is also off by allmost 1 tenth per mile. This my not be a problem for some. But I dont want a ticket and use my ex for work and charge mileage.

 

[addkev]

You are actually off by about 7% going from stock 235/75's to 31's. CBoug has this link to check your speedo change. You need to go to a ford dealer and get a different Speedo gear. They can help you figure your gear ratio, it costs about $18 if you do it yourself.

http://home.attbi.com/~cboug/Tire_Gear_Calculator.htm