One of the most requested changes to our cars is that people would like the stock analog speedometer to read higher than the stock 85mph (technically the ticks go to 95mph). This section will help you modify the electronics of your speedo to be calibrated for 140mph.
First a little background on the stock speedometer. Remember that these cars existed in the 1980's, when insurance rates were very high, and speed-rated tires were only available on a select few vehicles. Most vehicles from that era had speedometers that did not read past 85mph in the United States. Bowing to pressure, Ford did this with our cars as well (with both analog and digital speedos). Owners were pretty much stuck with the stock configuration.
However, there is an actual 140mph speedo that was available for our cluster. It was not available at Ford dealerships, but rather through an authorized Ford Motorsport dealer. The legendary Ford 140mph speedometer simply dropped right in—no calibration necessary, and no extra wiring needed. Brand new the 140mph speedo listed for $150 US. It's been discontinued since 1993, and until recently the going price of a used 140 mph speedo unit on eBay has been around $300 US. So if you want an actual, genuine Ford Motorsport 140mph speedometer unit, you will definitely have to reach deep into your pocket—if you can find one.
Recently a very informative message board thread brought up the possibility of converting a stock 85mph speedo over to one that reads higher. Through a collaborative effort, it was discovered that one resistor change is all that's needed to accomplish this. After the physical change, a new speedometer face is all that's needed to complete your conversion.
One item of concern is that the new speedometer can read lower than actual speed at speeds below 40 mph, remain fairly accurate from 40 to 100 mph, and then read higher than actual at speeds higher than 100 mph. The greatest amount of "drift" is about 10 mph, which means that the speedometer may only show 25 mph when the vehicle is actually traveling 35 mph. If low speed accuracy is a major concern for you, these speedos can be recalibrated with the use of special equipment, namely a wave function generator and a Hz meter. There is a large center weight on the armature of the speedometer that will either have to be moved inward or outward to take care of the low and high speed inaccuracies. It's also possible that a 1K resistor soldered in-line may also help correct it. If a feasible solution is found it will be posted on this page at a later date.
Now the physical changeover of the speedo from 85mph to 140mph is fairly simple. But by no means does it imply that your car will actually attain those speeds. This mod is simply to make your car have a simulation of a rare piece and does not warrant excessive speeding for usage!
This article was written and compiled by Joe—much thanks to him and to all the message board members who made this mod possible.
The speedometer must be removed from the cluster in order to do the conversion. Start by removing the six black cover screws. Once the screws are removed, you should be able to take off the clear cover and the black plastic cover. Now it's time to turn our attention to the back side of the gauge cluster. The speedometer is held in place by 6 screws on the back of the cluster, 3 gold screws and 3 tiny black screws. Be very careful when working—you DO NOT want to damage the printed circuit. Once you have removed all the screws, you can now remove the speedometer from the cluster.
Now remove the needle from the speedometer. It is pressed on the shaft, but should come off by pulling straight up on it. This may take a while, so be patient. Twist off the extension to the trip reset. Now remove the 2 black Phillips head screws on the speedo face. You can now remove the face.
To get to the circuit board a screw must be removed on the back of the speedometer. Once the screw is removed the circuit board should be free to separate from the odometer/armature assembly.
The capacitor that will need to be changed is marked "473K" in the "cv" position. It is not only soldered in place, but also glued to the board. Unstick it from the board. Using a soldering gun, heat one of the factory solders while pulling on the capacitor on the other side of the circuit board. When you feel the cap move a little, heat the other solder until the cap moves again. Keep alternating the solders until the cap works its way out. Take the new capacitor and put it in place of the old one. Heating the solder while pushing on the cap will allow it to get through the holes. Once the new capacitor is in far enough, determine whether or not the old solder spots need more solder. If they still look adequate, leave them be. Put the circuit board back onto the odometer/armature assembly.
NOTE: This section relates to completely changing the speedo face. If you wish to just cover up the old face with a new sticker, skip to "Printing the New Speedometer Face" below.
The first thing to do is remove the old face. Using a precision screwdriver, get underneath the old face and slowly pry the face up in various places. If you're having trouble with this part, placing the face over a pot of boiling water may help to soften the factory adhesive and allow for easier face removal.
Now it is time to sand off the old face to make way for the new one. By sanding off the old face, we can still use the factory green light diffuser for a more stock appearance (you can put the face on the back of a styrofoam plate to assist in the sanding process). Keep sanding until all the black and white are removed from the green disk. Clean all the sanding debris off the disk.
The printing and cutting out of the face is next. You can find a complete list of speedo and other gauge faces in the Downloads section of this site. Simply select which style of speedo face you would like to use and download it. Open it open in MS Paint, Photoshop or equivalent program and it should be the right size. Then simply print out the face on the special adhesive-backed paper.
Before handling the face, spray the clear paint on it, giving it a light, even coat. This will protect it from fading from UV light and from smudging while you handle it. Once the paint has dried, cut out the inside details on the face, then the outside perimeter. This way you have something to grab onto besides the face itself while you cut out all the little rectangles and circles. If you are making a black face, now is the time for the Sharpie. Use it to touch up all the "white border" you get from cutting out those rectangles.
Once you have the face cut out, peel of the cover for its adhesive backing, and very carefully center it on the green disk, making especially sure the odometer rectangles are lined up correctly. Slowly work the face onto the disk, starting from the center and traveling outward to prevent air bubbles. Apply some adhesive to the perimeter of the back of the disk, and put some around its center hole. Press it back onto the clear plastic plate and wipe up any excess adhesive that might squish out.
All that's left is to put the speedometer back together and to put it back inside the instrument cluster. Now go out and enjoy your 140 mph speedometer!