70 Challenger Speedometer is off

[Mc MOPAR]

Hi guys, the speedometer is off on my 70 Challenger. It reads around 85mph when I'm actually going about 60mph according to my GPS. I'm just wondering if it may have the wrong gear in the 727 transmission where the speedo cable threads into it? I've been told there are different colours. What colour should it be with 3:23 gears in an 8-3/4 rear end?

 

[Litchkar]

It is calculated by both the rear diff ratio and the tire diameter. The gear color represents the number of teeth on it. This chart will tell you the correct number of teeth for your application.

 

[tonysrt]

Your tire diameter is missing in the equation so it can't be figured out.

 

[moparlee]

Your tire diameter is missing in the equation so it can't be figured out.

Look at the chart. Tire diameter is the vertical column on the left and the gear ratio is the on the top horizontal line.

(Sorry I see you meant that McMopar didn't list his tire size. My bad)

 

[Mc MOPAR]

Sorry, I'm running BF Goodrich Radial TA's, 255/60/15 which have a tire diameter of 27.1"

 

[moparlee]

Sorry, I'm running BF Goodrich Radial TA's, 255/60/15 which have a tire diameter of 27.1"

From the chart you need either a 31 or 32 tooth gear. Your tire diameter is right between the two. The number of teeth are stamped on the gears.