At highway speeds, it's more noticeable. There's a noticeable discrepancy between the speed displayed in the Head Up Display and the analog speedometer. HUD always displays the lower speed by ~ 2-3 km/hr (~ 2 mph).

Which is the accurate one?

 

I remember the two being different too but I dont remember which one was higher and which one lower. I would think the HUD is more accurate but that is just guessing.

I will check tomorrow but I think the HUD was higher in reading than the speedo

 

I don't find much of a discrepancy, maybe 1 MPH at times. HUD is probably more accurate since the speedometer needle is slower to react. Or at least it gives that illusion. I'm not really sure because I typically drive below 45 MPH.

 

Since the speedometer has a mechanical needle, there is some play that exists there that will not exist in a purely digital system. Still, most interstate expressways have mile markers, accurately placed. With a passenger, a stopwatch and 6th grade math (maybe it's high school math today), you can measure the speed of the car yourself (take several readings), and compare that reading to what your auto instruments are telling you.
It's pretty much the same way we check our miles driven against actual gallons of gas pumped into the car to determine how accurate the MPG readouts are from the car's computer (pretty accurate).
Here, we can only generalize. With your own measurements, you can answer the question with assurance with respect to your vehicle.
Let us know what you find out.

 

Your way of measuring correct speed with mileage markers and a stop watch is spot on. But unless I'm sadly mistaken, everything in the dash is electric and no more cables (I'd bet a dollar to a donut there is nothing but wires going to the speedo head). That's why I'm surprised there is any difference at all. But I don't have the HUD (sure wish I did) so I can't speak to that.

 

Jim,
The needle spinning around on the dash is mechanical, even if the data gets there electronically. (The big fat speedo cable is long gone.) So what I am saying, based, as is often the case, on no information whatsoever, is that there surely is some play, no matter how small, in that mechanical system, as opposed to an LCD or LED readout, which would be more precise. Surely both take their data from the same sensor under the hood (or wherever), so the only place for a difference to appear is in the mechanical system. If the needle is above the printed numbers, parallax itself would cause different readings from different viewpoints.
All in all, it's maybe 1 or 2 MPH, so it's a fairly academic question anyway, but my guess would be that the digital readout is the more accurate of the two. Whether that readout is absolutely accurate can be measured, as noted, for each individual car.
Merry C.
I gotta go.

 

+2 points for using parallax correctly in a sentence. +1 more for spelling it correctly. +2 more for the usage not being an astronomy forum. 5 points, full marks.

 

Thank you so much. I did take an astronomy course at Georgetown University, so maybe something stuck. Still I am not a scientist, and frankly I thought everybody knew what parallax was, especially after the success of "The Parallax View."
Perhaps that movie should be required viewing here.

 

Agree with the scientific explanations and I refrained from commenting on this to not sway anyone on the forum one way or another. I come from a science background myself and had similar conclusions that the HUD would be the accurate one hands down... but...

I wanted some form of official answer from Lexus as I couldn't rest there. I asked my dealer on which is accurate and got a response today. To my surprise, the response in an email stated, "After getting feedback from three technicians, the SPEEDOMETER IS ACCURATE". Stumped that the implication to this simple statement is that the HUD is the lesser accurate gauge, I requested technical details as to why and how the speedometer and HUD operate to explain this. I'll post any response I get. To be continued...

 

I have some bad news for you, corradoMR2. As Heisenberg's Speedometer Uncertainty Principle states -- you can either know the speed you are traveling via the speedometer or the speedometer's accuracy, but not BOTH.

 

Of course, Lexus will say the speedometer is accurate, but that was not the question. My guess is that they do not want to say in writing that the speedometer is less accurate than anything else available, especially when it is accurate enough.
As for the HSUP, I agree completely, but also think his cat is dead.
Merry Christmas.

 

I believe the variance on mine is infinitesimal.

 

I have a portable TomTom Nav that tells exact speed triangulating 3 sats; It always told me exact speed I was drving and what the speed limit was on top of each other; it would confirm as to which is more accurate(wish Lexus had this on their Nav)!

 

As to the comment by WRX: "I believe the variance on mine is infinitesimal," I would say that's exactly the point of this whole diverting discussion. We are talking theory here, because even though we all seem to think that a totally digital system would be inherently more accurate than one with a mechanical interface, with the sophisticated engineering and small tolerances of the modern speedometer, the difference, when observable, is so small as to be negligible.
I love this tech talk.

 

What is the mechanical, electromechanical, analog or digital means speed is determined before being represented in a speedometer or display?