Calibrating Speedometer

[Dangerous Fish]

The radar traps are not giving an accurate reading (often they will over read on purpose to make you drive even slower than the limit).
The speedo in a Model 3 is within +/- 1mph of actual speed.
I know because I've got all sorts of fancy data logging equipment I regularly put in them for testing.

 

[cat7]

I'm pretty certain that modern cars have *precise* speedometers already. The inaccuracy is clearly *intentional*. The problem isn't the ability to do it. While i don't know this first hand I'm pretty sure this is the case of the lawyers/regulators "winning". There is effectively zero incentive for an auto manufacturer to remove the bias that they coded/built-in. This point was made by several posters before me. I'm just here to beat the dead horse.

There will always be some level of error on the measured speed, even if calibrated with GPS when travelling at a constant speed every so often. So the car makers must bias the speed shown to the driver up a bit to make 100% sure there no chance the speed will ever be shown lower than the true speed. If the car maker can guarantee a ±2mph accuracy across all vehicles manufactured in all conditions (tyre pressure/wear, temperature, tolerance in mechanical and electrical components etc) then at minimum they will need to add 2mph to their calculated speed to show to the user. In reality they probably add a little bit more than they need to just to be sure.

 

[Robert A]

While there is some propension to over-report as many have indicated, tire wear also affects the reading and makes it hard to be very precise. Does tire size and/or tread wear significantly affect speed and speedometer accuracy?

Granted we're talking a couple percent but at highway speeds that's more than 1mph difference.

Yes, but tire wear increases the overread because worn tires spin faster. Still, if the GPS system is periodically recalibrating the speedo, this ceases to be an issue.

 

[metroplex]

Is there a way to recalibrate the speedometer?
On my Fords as far back as 2007 can have the speedometer recalibrated through the ECU thanks to electronic speed sensors (versus the old gear). I have found some of my newer Fords were spot on. I just had to change the ECU setting for my 2017 Fusion Sport because of the diferent tire size. With the stock tires, the vehicle speedo matched my 10 Hz GPS speed reading. Once I made the tire size change and changed the ECU setting via FORScan, the actual speed matched the speedometer. My 22 M3LR is reading 1 mph higher than actual on the freeway.

 

[Candleflame]

Is there a way to recalibrate the speedometer?
On my Fords as far back as 2007 can have the speedometer recalibrated through the ECU thanks to electronic speed sensors (versus the old gear). I have found some of my newer Fords were spot on. I just had to change the ECU setting for my 2017 Fusion Sport because of the diferent tire size. With the stock tires, the vehicle speedo matched my 10 Hz GPS speed reading. Once I made the tire size change and changed the ECU setting via FORScan, the actual speed matched the speedometer. My 22 M3LR is reading 1 mph higher than actual on the freeway.

yes, and so it should. speedos are not allowed to underread and teslas have the most accurate speedometers which exist. (1km/h above with new tires usually).

 

[metroplex]

yes, and so it should. speedos are not allowed to underread and teslas have the most accurate speedometers which exist. (1km/h above with new tires usually).

So if Tesla reading speed 1 mph faster than actual at 70 mph is considered the most accurate speedometer that exists, then my Ford speedometers are precision laboratory-grade speedometers (speedometer reading matches actual speed confirmed with aviation-grade 10 Hz GPS).


I'm going to change the wheel/tire setting in the Model 3 to the Zero-G 20" for the 245/35R20 size to see if it fixes this. I believe based on the math it should fix it, or get it very close.


I just don't understand why Tesla offering so many ways to configure the vehicle within the computer, doesn't have a way to correct the speedometer. I was able to do this rather easily on every vehicle I've bought since 2005.

 

[AdamVIP]

I would love an option for this as Im not too keen on the stock 135 tire size with the Uberturbines and the next exact diameter size is too big to run.

 

[metroplex]

Just wanted to report back on changing the wheels to the Zero-G 20"... it had zero impact on the speedometer. Still indicated 1 mph faster than actual. Sigh.