’14 IS250…

My grandson was messing around with a speedometer app on my phone, and we noticed a discrepancy, so I researched further…

I’ve got a couple Garmin eTrexs, and set my cruise control to exactly 80 mph on level ground, no wind. Both GPSs read the same, 77.22 mph.

I do have the 18” wheel upgrade, but the tire size is speced correctly.

I might see 1, maybe even 2 mph difference from actual, but 2.78 seems a bit much.

is there any way to adjust/calibrate the speedos in our cars?

 

Almost all cars speedometers cannot be 100% exact due to several factors and federal law allows for 5% variance. Most cars read a little bit high (to prevent speeding / tickets), if you car was reading slower than actual speed then that would be more cause for concern.

Here are some articles explaining a bit more in detail

https://www.roadandtrack.com/car-cul...ters-accurate/

https://www.foxnews.com/auto/how-fas...f-speedometers

 

+1, every car I've owned has registered 2-3 MPH higher at 65 MPH than what is indicated by those "YOUR SPEED" signs on the side of the road or the speed reported by Google Maps.

 

Also the faster you go the greater the discrepancy. At 60mph it's probably 1-2mph off.

 

In my case the cluster shows approximately 11% more than the actual speed (in km/h).


The reading on the center console is a real close match to the GPS speed reading on my phone.

 

Hey, couple questions for you…

How is it you’re going 145 kph, with a gate right in front of you?
Also, do you know from where each speed is referenced from?

Lastly, but not related to speed, your battery voltage seems very low. A running engine should be between 13.4 and 14.2 vdc.

 

Hi,
I've put supports under both left and right swingarms, so the running gear was in neutral position, without the wheels touching the ground.
Regarding the references the center console reading is originated from the transmission output shaft sensor. I'm just guessing in regard of the
cluster, probably it gets the reading from the CAN bus, and "distorts" the results.

The 12.6V voltage reading is normal when the battery current drain drops below a predefined threshold in this case the alternator voltage output is reduced.
When turning the headlights on the 12.6V output is instantly jumps up to 13.6V
14.4 - 14.5 output voltage is present only when the battery needs to be topped up (mostly after starting the engine).

 

Wow, 145 kph on jack stands. You got bigger ***** than me. 👍

What is the CAN bus, and why would it be different from the tail shaft sensor?

And just another question, how did you compare the console screen speed with a GPS if the vehicle was not moving?

 

Basically all sensors, switches and actuators, control units are wired up to this CAN bus.
This is a standardized serial data bus for data transfer between various units in the vehicles.
So the shaft sensor output is available to all units connected to the bus (including the instrument cluster),
of course many of these units doesn't need or require that info.

The console screen - GPS compare was took place in a different round, under real road conditions.

 

Maybe I wasn’t clear to my question…why would a vehicle have two different speed reference sources?

What’s the point, and why would they be different?

 

From sensor point of view I'd say there's only one speed signal source exists.
Providing higher readings for the driver is supports the "to be at the safe side" theory mentioned earlier,
while the vehicle stability systems, and the transmission uses the real data.

 

So this is your assumption, correct?

So manufacturers “add” to the indicated speed, even tho there is more accurate data available, because they want to protect us from tickets??

Maybe it’s just me, but I still don’t get this…I would prefer the accurate data to be displayed…I’m a big boy, I don’t need “help” to keep me from getting tickets.

Oh well, thanks for the replies. 👍

 

Yes, because using two sensors would mean additional costs at the production,
and today it is more easy (and also cheaper) to produce different readings from one signal source.

I agree. Optionally the accuracy should be configurable by the user. If someone wants to add a custom percentage to the actual value well let it be,
if not, setting the accuracy to 100% would display the real speed.