I have 2000 Ranger 4WD with different rear gear ratio than the original. (Done by some idiot mechanic of previous owner)
I learned that in 2000 models speedometer works by using wheel speed sensor mounted at rear differential.
If so, speedometer should show correct speed no matter what ratio differential is used (with same tire size of course). Am I right?

To confirm that I compared it with GPS on my phone.
Comparing with speedometer there is about 4MPH difference. Comparing with OBD2, there is about 2 MPH difference, which I think is acceptable error. So, it seems the needle is little off.

Am I correct on my diagnose?


Also, I have issue with ABS kicking in when stopping. Checking ABS DTC, it showed no signal from rear wheel speed sensor. But that makes no sense because when checking ABS speed sensors, it had signal from all 4 wheels. Since there is no separate sensors for left and right rear wheels, I think both rear wheels get speed from same sensor on rear differential.

Could it have anything to do with mismatching gear ratio?

I plan on trying cleaning front ABS sensors first, but cannot seem to find enough time to do so.

 

Yes you are correct and your speedo is well within spec for a non police vehicle.
Your ABS problem isn't uncommon, you could have a wiring issue(corroded connectors or a broken splice) or it could be dirty sensors, but it has nothing to do with mismatched gear ratios. If you have a wiring problem it is not at the rear axle it's more likely to be at the ABS controller.

 

Your comments make me wonder if the front/rear ratios are the same???

 

I figured out what was the problem by using scan tool to monitor sensor inputs. Right front had no input below 10km/h (tool was set to km, didn't bother changing it to miles).
So, I though let me try to clean it. After fighting through all the rusts, I finally go to the sensor. Then broke it trying to remove. Outer plastic shell is stuck inside hub from the rust.
I'm planning on changing whole hub.


They are not the same. Making 4WD unusable. That is why I'm calling the mechanic who changed rear axle idiot.
How much do the rear axle costs at a salvage yard? If it is cheap enough and I come to like the truck I might change it out. Until then, 2WD only.

While removing front right caliper, I found more mistake. 2 anti-rattle shims were installed at the bottom, and none installed on top. One of the slide pin was completly stuck with rust. So, guess it was a good thing I disassembled it.

 

Just replaced hub assembly today and no more ABS issues.

I tested GPS speed vs OBD II speed on highway with Torque and looks there is average speed difference of 4 MPH and on the actual gauge it is about 5 MPH or more. Is this really within the spec? Seems pretty big difference to me.
Anyway to calibrate this without costing too much?

 

4-5 mph error is fairly common. Varying tire diameters (from stock), even among those with the same printed size, account for a lot of the error.

Most of my four wheel vehicles are accurate to 3-4 mph, but the motorcycles I've owned are commonly 5 mph off even when new.

 

The tire size is stock size and probably have 70% of tread. My other vehicle (Toyota Highlander) is very accurate within 2 mph or less. Haven't tested on the other vehicles. Never knew that much error is common.
At least the gauge is showing faster than it actually is to avoid tickets.

I tested ABS sensor speeds from all 4 wheels (although there are only 3) and they all shows the same speed as OBD II data. Which confirms it has nothing to do with gear ratio and the speedometer data is from the ABS sensor.
Maybe I will try to calibrate the needle little bit when I remove the instrument cluster. One of the bulb is out, planning to changing all of the bulbs. (May get LED?)