Model Y speedometer error

[SSonnentag]

The speedometer in my MYP reads 2% low vs. GPS. Does anyone know whether the speed is read from the front or the rear wheels? The front tires have a 0.6 inch (15 mm) smaller diameter than the rear tires. I'm wondering if increasing the front tires by 2% to match the size of the rear tires will correct the speedometer error, or do I need to increase the rear diameter even further?

Front Tires: 28.0 inches
Rear Tires: 28.6 inches
Diameter Difference: 2.14%

 

[GtiMart]

Have you configured the car with the proper wheel size? Since you have a different diameter front and back, is one of them running the stock overall tire dimension?

Smaller wheels would make more turns for the same distance so they would indicate faster, not slower.

 

[Dangerous Fish]

Vs. what GPS? The car's? Your phone? A tracker? How are you making the comparison?
Assuming you have the stock 21" Uberturbine staggered set, the difference in revolutions per mile is 1.
Even assuming an error of 2% that's 1.2mph at 60 mph.
I don't see what the issue is?
Most GPS modules are only capable of accuracy to around + or - 1mph anyway.

 

[SSonnentag]

Have you configured the car with the proper wheel size? Since you have a different diameter front and back, is one of them running the stock overall tire dimension?

Smaller wheels would make more turns for the same distance so they would indicate faster, not slower.

The stock tire sizes are the MYP are what I'm running. They're different diameter front and rear.
Speedo reads high. When speedometer reads 55 I'm doing 54. At 75 I'm really only doing 73ish.

GPS on phone for comparison, which is extremely accurate when using cruise control on straight sections of road in the open Arizona desert, within 0.5% margin of error and typically within 0.2%. I compare against a GPS device with sub-centimeter accuracy.

 

[SSonnentag]

Vs. what GPS? The car's? Your phone? A tracker? How are you making the comparison?
Assuming you have the stock 21" Uberturbine staggered set, the difference in revolutions per mile is 1.
Even assuming an error of 2% that's 1.2mph at 60 mph.
I don't see what the issue is?
Most GPS modules are only capable of accuracy to around + or - 1mph anyway.

It's not really an issue, it's just that I'm switching from the 21" wheels to 18" and would like to figure out which tire size will give me the most accurate speedometer while I'm at it. There are a few different tire size options with minimal tire diameter variations of about 27.9-28.6 inches.

 

[MP3Mike]

So which is it? Does the Speedometer read low:

The speedometer in my MYP reads 2% low vs. GPS.

Or does it read high:

When speedometer reads 55 I'm doing 54. At 75 I'm really only doing 73ish.

But the regulations state that the speedometer can never underreport the speed, so Tesla purposely always reports it approximately 1 MPH higher than actual. There is nothing you can do about that.

 

[Dangerous Fish]

The stock tire sizes are the MYP are what I'm running. They're different diameter front and rear.
Speedo reads high. When speedometer reads 55 I'm doing 54. At 75 I'm really only doing 73ish.

GPS on phone for comparison, which is extremely accurate when using cruise control on straight sections of road in the open Arizona desert, within 0.5% margin of error and typically within 0.2%. I compare against a GPS device with sub-centimeter accuracy.

Are you saying you used your phone to compare with the car and you previously compared your phone with some other device? Which GPS device is/was that?

 

[mcsenerd]

....

But the regulations state that the speedometer can never underreport the speed, so Tesla purposely always reports it approximately 1 MPH higher than actual. There is nothing you can do about that.

This...

I can't remember when or if I've ever had a stock vehicle that did NOT underreport speed, likely for this very reason.