Speedometer Drive Gear, Stock Dana 20, 66-77 Ford Bronco SKU: 7101

Early Bronco Speedometer Gear

The early Bronco speedometer drive gear is a nylon gear that mounts to the end of the speedometer cable to ensure the speedometer on your 66-77 Ford Bronco reads accurately based on the differential gear ratio and tire size you are running. These speedo gears are available in 16 tooth to 21 tooth variations.

Below we have provided detailed information for calculating which speedometer gear you will need for your early Bronco. Aside from using the formulas provided, a common way of identifying the best speedometer gear for your Bronco is to first identify how far off your speedometer is currently reading as a percentage. Look at your current speedometer gear, and count the teeth. Each additional tooth will make the speedometer read about 5-6% slower. For the speedometer to read faster, reducing teeth will accomplish this at the same rate. So, if you figure your Bronco speedometer is reading about 20% too slow, you will need about 4 fewer teeth on the new speedometer gear.

Speedometer Gear Calculation Formula:
The following formula can be used to calculate which speedometer gear you will need for your 66-77 Ford Bronco or other vehicle based off of the differential gear ratio, the tooth count of the transmission drive gear and the size of tires being run.

D = # Drive Teeth
G = Axle Gear Ratio
R = Tire Revolutions per Mile

D x G x R / 1001 = # Speedo Gear Teeth

To calculate the Tire Revolutions per Mile for the above formula, you will need the following calculation:

(Tire Diameter) / 0.20168 = R

Below, we have put together a chart of the common early Bronco gear ratio and tire size combinations for a quick reference when purchasing a speedometer gear. This chart is based on the typical early Bronco 7-tooth speedometer driven gear.

Early Bronco Speedometer Gear Calculation Chart

What to do if I need a speedometer gear tooth count that is not available?

If you have done the calculations or looked at the chart and found that you need say a 14 tooth or 24 tooth speedometer gear, there are additional options to get your early Bronco speedometer dialed in.

Many early Bronco drivers opt to get the speedometer as close as they can with a 16 tooth or 21 tooth gear and then figure out the percentage the speedometer is off and take a mental note. This is adequate but does require you to constantly be doing math in your head while driving to ensure you stay within the speed limit.

The best and simplest option may be to convert your factory speedometer gauge to a new digital gauge. Many modern digital gauges allow for programming of the speedometer to account for varying tire sizes and gear ratios. Some also support gathering speed information through a GPS antenna.