Speedometer Problems - Inaccurate

[tomeeg]

I just upgraded? from a 92 to a 97 Explorer Sport today and noticed the speedometer appears to be off 10 +/- MPH at all speeds. As I read threads on the site, many responses point towards the speedo gear. I have the stock tires on mine (23575R15), so I'm wondering how does the speed all of a sudden be off because of a gear when I haven't changed anything from stock or should I be looking elsewhere for the problem? Is this normal wear & tear (90k miles). It is a 4.0L/Auto/4WD if that helps. Any info would be appreciated.

Tom

 

[Robert]

Maybe the person that had it before you used to run taller tires and changed out the speedometer gear? If you bought it from a dealer, maybe they put the stock sized tires back on or maybe the previous owner put stock size tires back on since they would be cheaper and they possibly knew they were going to get rid of it. If you are actually 10 MPH off at all speeds, then I would more likely suspect a problem with the speedometer itself rather than the speedometer gear. The speedometer gear will change your speed linearly. Each change in your gear by one tooth amounts to about a 5% change. If you change your gear to one with 2 more teeth, your speedometer will read approximately 10% slower. In other words at 10 MPH, your new indicated speed would be 9 MPH. At 70 MPH, your new indicated speed would be 63 MPH. For it to be an equal 10 MPH across the entire range, I suspect your speedometer is faulty. How does your odometer perform? Try measuring between the mile markers on a highway (the more for your average, the better). If your odometer reads accurately, then it isn't related to your speedometer gear since it is driven off the same signal. Any chance you have aftermarket gauges (white face gauges perhaps)? If you do, the person putting them in, probably didn't get the needle back in the proper position.

 

[jimbo74]

not 100% sure, but i think that only the 91-94 had an actual speedo gear that played with the guage.... i believe after 94, the transmission changed to an electronically controlled unit and also the way of measuring it changed to.. i htink it moved from the tranny to the tcase on a 4x4?

 

[Robert]

Nope, Ford still used a speedometer gear mounted on the tranny tailshaft for 2WD or the transfer case for 4x4 up through '97. I've changed my '97 speedometer gear three times now. Beginning with the '98 model year though, Ford started using the rear ABS sensor to measure speed and did away with the tranny/tranfer case VSS.

 

[unclemeat]

There are GPS speedometers available now. Most GPS handheld units will function as a speedometer. This may be the simplest fix.

 

[tomeeg]

Looking a little closer at it, it appears that the needle may be off. At a stop, the needle still rests above the 10 MPH mark slightly. I did check it tonite at a Police radar unit and at 51 MPH, it recorded 41 MPH, so exactly 10 MPH off.

It looks like access to the speedometer is through 3 screws on the top. However, isn't the speedometer in a sealed unit or does the clear plastic face to the gauges come off also to access the needle. I'm guessing that the needle needs to be pulled off and reset.

Thanks for the replies.

Tom

 

[yob_yeknom]

You have to take the dash apart to get the cluster out. The clear cover can be removed from the cluster once you get it out.

 

[tomeeg]

Still never figured out the speedometer problem. I did determine I have a 20 tooth black speedometer gear, but from reading posts, 21 is the highest number of teeth available and if that helps 5%, I would still be 7 or 8 MPH off at 41 MPH. I did not the odometer is not reading accurately either. Is the guess still a bad speedo?

 

[olemech]

speedometer

i just put 13x10.5 on my 1996 explorer 4x4 nad had to change speedometer gear in trans. to a 18 teeth. now speedometer is back reading the right speed and milage.