Was inspecting my decrepit tires today and noticed what appears to be an electrical sensor on top of my rear differential, and my crappy haynes manual doesnt identify it... anyone know what this is, why its there, and if all SD's have it?

 

It's your VSS. All SD's have them.

 

Now aint that interesting... now on the topic of VSS, with 33's on my truck my speedo reads about 4 mph too slow (it says i'm going 55, my tomtom and police speed signs say i'm going 51) could that indicate a problem with that sensor or does everyone's truck do that?

 

Anytime you change tire size it will effect the VSS reading. You can have it recalibrated but IMO isn't worth the cost (tuner, factory recal etc..) for a few MPH.

I typically figure at least a 10% accuracy factor between anything.

 

yes that is correct, the reason I'm concerned is that usually when you increase tire size, the actual speed is FASTER than the speedo reading, not slower. Thus, if I put stock tires back on my truck the speedo discrepancy would be even further off than what it currently is. This makes me wonder if the PO may have had the VSS calibrated for a much larger tire like a 35 or 37, but i am not sure if this is normal or not.

 

To my knowledge the calibration is on the car computer and has nothing to do with VSS.
I just ordered the sensor, since my seems to be acting. I do have ABS light on and occasionally speedometer is dead. Can't figure out how-come speedometer can still work correctly most of the time, when ABS taking signal from the same sensor is completely dead, but that how it goes.
VSS is probably the same technology what CPS is and fail as often, although it will not immobilize the truck.
What also makes me wonder, if that is the same technology how-come I bought new VSS for $17 (ebay) when CPS cost several times more?

 

I have 33s on my truck and did nothing with the speedo. It is actually more accurate now than when I had the stock tires on.

 

No. Larger tires would be fewer revolutions for the same distance, indicating a slower speed than actual.

 

Here I'm not sure if we're saying the same thing or not... if the vss is measuring revolutions in the differential it is going to see X amount of revolutions per time is X speed, if you put larger tires on the same amount of revolutions the vehicle would be going faster than the speedo indicated... such as it would say you're going 35 (with stock tires) and you'd really be going 40 or so (my roomates chevy with 37's is actually traveling 30% faster than the speedo indicates over his stock 245's)

In my case the speedo says I'm going faster than I really am, which is counterintuitive to the contrary of the happenings of bigger tires, by which my speedo says 35 and I'm really going like 29 or so.

 

I was assuming that if the speed is measured at the axle, there will be fewer axle revolutions for the same distance with larger tires.
But since I don't know exactly how SD's measure speed, I can't really argue your point. I DO know that my wife's son has an 04 SD with 37's and when we travel together, I'm normally the pace rig since I have stock wheels & tires and my speedo is much closer to actual than his is. He reads low by more than 5mph. To that end, his (or anybodys) odometer is becoming more inaccurate (with fewer total miles) the longer he drives it with larger tires.

 

The VSS measures the revolutions of the axle. Increasing your tire size will make your speedometer read slower as the larger tires (and the axles attached to them) will have less revolutions per mile.

This is why you don't need to change your speedometer if you regear a differential.

 

The larger the tire the faster you are actually going compared to the speedometer. say you go from a size 30 to a size 33 that would mean that the speedometer will be off by 10% do to the 10% increase in tire size. so traveling 60 your actual speed will be 66.

 

You will find that on Ford from 87 to now.


My speedometer isn't accurate.

But notice it accurate at 45 mph but once you go 60 but gps say 57

 

my speedometer is dead on at max tire pressure (90 psi) and about 50% thread.
With better thread the speedometer was showing 60, while GPS 61.

 

I think most of you are saying the same thing regardless of perspective.

(true ground speed > than speedometer indicates) IS SAME AS (speedometer reading < true ground speed)