Accuracy of 'new' tachometer

[Doug_G]

I was thinking of saying the same thing but resisted because I realized that 'voice coils' can 'cheat' a bit. There could be track marks on the platter so the heads could watch for a data pattern and make the voice coil seek in and adjust heuristically to the correct position. On the other hand, old VOMs (Volt/Ohm meters) tended to have a coil to direct the indicator needle and they seemed reasonably accurate.

Yes, exactly; feedback can make almost anything accurate. That's why most analog electronics are built using op-amps.

The old VTVMs were pretty accurate, but the meter mechanism was fairly expensive and pretty delicate. It also needed a fine adjustment for zeroing the dial, which needed to be tweaked every time you used it.

Stepper motors also have the advantage of being "digital", in that they move in discrete steps. They avoid having any analog circuitry.

 

[TEG]

...Actually, speed and power output are values that do not really need to be so precise. Speed is often overestimated, as mentioned...

(non GPS based) Speedometers tend to be configured for an exact tire diameter. If you switch tires it can make the speedo wrong. Also as your tires wear down it changes the ratio. So, yeah, many speedometers can be inaccurate.

 

[Doug_G]

(non GPS based) Speedometers tend to be configured for an exact tire diameter. If you switch tires it can make the speedo wrong. Also as your tires wear down it changes the ratio. So, yeah, many speedometers can be inaccurate.

Yeah, but that doesn't explain the Roadster's speedometer. The car measures the tire diameters when you use the "new tire" function. The VDS display shows a very accurate speed, as verified by comparing against GPS*. So the car knows your speed very accurately. But the speedometer needle consistently reads high.

I wonder if all car manufacturers deliberately make their speedometers read high, maybe to avoid lawsuits over getting tickets? As far as I can tell, every car I've ever owned read a little high. My G37 does; when in adaptive cruise there's a digital display that is about 3% lower than the needle. The adaptive cruise display matches speed reported by GPS.

*I recently compared four methods of reading speed. Separate Garmin GPS, built-in Pioneer GPS, VDS, and speedometer. The first three all agreed; the fourth was high.

 

[TEG]

My old Ford RangerEV read slightly high too on stock tires.
But then I put on slightly larger diameter tires and now it is accurate.

Intentionally reading high to avoid speeding tickets??? Interesting theory.

 

[vfx]

My old Ford RangerEV read slightly high too on stock tires.
But then I put on slightly larger diameter tires and now it is accurate.

Intentionally reading high to avoid speeding tickets??? Interesting theory.

Which is why you can safely drive 57 in a 55 zone. ?

 

[zack]

My electrical engineering background is being re-invigorated by your spirited discussion about all of these possibilities. Thanks, everyone. It's interesting to know that analog and digital (stepped) approaches have both high resolution and high accuracy these days. I loves me my analog stuff.

Regarding speedometers, much like the way that produce sold in 1 lb containers is always just a bit high in actual weight, it only makes sense that auto manufacturers would tune their speedos a bit high. It's doing all of us a favor... especially us Roadster owners who, by and large, would have a tendency to push the limit just a bit. I mean, why did we buy this car anyway? Is it to avoid lawsuits? Or to do us a favor? Or did it simply start as a result of not knowing how accurate or repeatable any given speedometer would be, and choosing a safe point where the vast majority of speedometers would reflect either over or at the correct speed, and then as the years progressed and the meters became more accurate, the standard simply continued out of habit?

 

[Nik]

My old Ford RangerEV read slightly high too on stock tires.
But then I put on slightly larger diameter tires and now it is accurate.

Intentionally reading high to avoid speeding tickets??? Interesting theory.

I believe that UK (and therefore EU?) tolerance for speedo accuracy is to read true speed +10%/-0%.

Other benefits for the auto maker to read the speed high is that a fixed-mileage service interval comes around sooner, and the owner believes that they get better mpg as they're over-estimating their mileage.

 

[Lloyd]

I complained very loudly to BMW about the accuracy of the speedometer. Their excuse: "it was designed that way for SAFETY". I told them that for $100K+ car, German engineering should be able to make an accurate speedometer, and that being inaccurate in any direction is unsafe!

 

[cinergi]

FWIW, my Roadster's speedometer is less than 1 MPH off from the readings on the Alpine and the digital readout on the VDS at all speeds ...

 

[Doug_G]

FWIW, my Roadster's speedometer is less than 1 MPH off from the readings on the Alpine and the digital readout on the VDS at all speeds ...

On on road trip on Friday, I noticed that the speedometer was almost exactly the same as the VDS. First time I've ever seen that. I've also recently seen it read 10 kph high. It seems to wander around a lot.