I just picked up a 1996 Ford Ranger the other day. I noticed the Speed shown on the instrument cluster was very fast as I don't believe I was going 70 in a 55. I used a gps speed app and it was about 10 - 15 mph off. It wasn't jumping around or inconsistent. The odometer is still spinning and from what I understand it is the correct mileage but its almost to close to tell. Anyone know what speed sensor could cause this? I read someone had issues with it jumping around or no speed at all but this is consistently over by 10-15 mph. I apologize if I have this in the wrong place as I am new to the forum.

 

Welcome to the forum

The 1983 to 1997 Rangers use a speedometer gear on the transmission tail shaft, or transfer case(4x4)

This gear can be changed if tires size is changed or rear axle ratio is changed
1995-1997 Rangers still use the gear drive but with a VSS(vehicle speed sensor) instead of speedometer cable

You need some info to calculate the gear you need for accurate speed
DRIVE GEAR is 7 tooth<<part of transmission
Rear axle ratio ???
Tire diameter ???

With these 3 numbers you can calculate the new DRIVEN gear tooth count
You just need to remove 1 bolt and can pull out the VSS and DRIVEN Gear, it pulls off and you can install the new gear

Go here and get the rear axle code from your drivers door label: https://www.therangerstation.com/tec...le_codes.shtml
It will be a 3 digit number like 3.45, 3.73, 4.10, ect.....

Then look at your tires, get the Metric number from the side, i.e. 235/70R15
Enter that number here to get diameter: https://tiresize.com/calculator/

Previous owner may have swap rear axles so ratio might not be what it came with, as stated on label
Tire size is almost never what it came with, lol

 

OK. Thank you for the help!

 

Forgot the gear calculator, lol: https://www.tciauto.com/speedometer-gear-calculator

If calculation comes out as 18.7 then use 19 tooth gear, if 17.3 then 17 tooth gear
Ford gears come in 16 to 21 tooth

And it might be the VSS is bad but usually these fail out right, no speed at all, not inaccurate speed

The VSS unit looks like this with gear attached, gear comes off: https://www.therangerstation.com/tec...s_withgear.JPG

The previous owner may have had larger tires and swapped gear out for that, and decided to keep the tires before selling and put back on the smaller ones but didn't change the gear back

 

I have a 97 2wd 4.0 supercab w a 5r and the trans went and I replaced it with a used 5r from a '99(sticker on the bellhousing says 98 tho for some reason) and ever since I did the swap my speedometer has read slow, and the faster you go the slower it is, at 55 it reads 45, at 60 it's 49, at 69-70 it says 55, 85 says 65 and when you're doing 95 it says 75, that's the fastest I've checked it, and in city speeds its 5-10 off, you have any idea what the problem could be bc I'm stumped and I haven't been able to find anything online

 

1998 and up 2WD Ranger transmissions don't have the speedometer cable hook up(hole) on the tail shaft housing
So I assume you swapped tail shaft housing from the 1997 to the new/used transmission?

Automatics have the Speedometer Drive gear machined on to the tail shaft so it would be there you just need the "hole" to access it
And machined Drive gear can't "slip", and I assumed you would have noticed if this gear was damaged

Could be a couple of things
VSS gear drive is slipping, check odometer on mile markers, if its also off then replace driven gear and VSS unit
Speedo and odo share the 1 signal from VSS sensor

If odometer is working then issue will be in the instrument cluster, speedometer motor

Extreme long shot would be Alternator leaking AC voltage, AC Ripple
The VSS generates and sends an AC Voltage signal to the speedometer/odometer, the Sine wave AC pulse per minute is the speed not the voltage
Alternators generate AC voltage, and its converted to DC Volts inside the alternator, a failing diode can cause AC voltage to leak into the vehicles electrical system and cause odd issues with speed and cam sensors
You can test for AC ripple with any Volt meter, instructions on-line