Speedometer off by -1 MPH

[FlasherZ]

At least it's not like my GTO's speedo. There you have to average the two speeds it shows as the needle oscillates between the two (e.g., if it bounces between 30 and 35 mph, you're going 32.5 mph).

 

[N4HHE]

Well - according to German law the speedometer can only have a tolerance to the high side. It can never show a speed lower than the one you are driving.

That applies to all of the European Union. Its silly IMHO because it teaches drivers the instruments are lying and one should drive even faster to compensate.

 

[ken830]

For the last dozen years, I've had to link to this Car and Driver article on every car forum I've ever participated in:

Speedometer Scandal!

Basically, most speedometers read high. In general, German/European cars read the highest, followed by Japanese/Asian, and American are generally pretty spot-on. However, the trip computers and odometers/etc, don't have such a conservative buffer of reading high in its calculations. But then again, this was written 12 years ago and thing may have changed.

 

[jcaspar]

US law is similar.

PS The Prius is a big fat liar on speed and fuel economy.

Looks like the Prius discussion groups have confirmed this:
Calibration of 2010 Prius MPG indicator - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

And suggest this conversion formula:

• use correction formula: actual = (.85 * indicated) + 5.3

Or for a Prius, about 10% less than what the car tells you.

 

[aviators99]

Mine is exactly on, when it is stopped!

On my 2007 Lexus LS460L the car reads about 3 mph fast for speeds above about 20 mph. In the Lexus group there was talk about challenging Lexus about their warranty, since it is both time and mileage based and the odometer shortchanges the warranty on mileage. No issue with unlimited Tesla mileage.

Tesla warranty is 50,000 miles, except drivetrain and battery.

 

[jerry33]

Looks like the Prius discussion groups have confirmed this:
Calibration of 2010 Prius MPG indicator - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com

And suggest this conversion formula:

• use correction formula: actual = (.85 * indicated) + 5.3

Or for a Prius, about 10% less than what the car tells you.

I don't buy that at all. I drove my 2004 Prius for 146689 miles and put in 2587.37 gallons of gas, giving a 56.7 mpg. The MDF mpg shows 58.8, giving a 1.51% difference. My thought is that anything within 5% is the same as zero given the variances in the accuracy of the measuring devices used.

If they only did one tank to get their numbers, the value can be out by quite a bit because it only takes a few ounces difference in fuel level when filling to get a material difference in the calculation. You need at least ten tank fills to get a reasonably close approximation, and fifty fills starts to approach accuracy.

 

[evp]

Airplanes usually have tables in the manual relating "calibrated airspeed" to "indicated air speed", due to unavoidable differences in how the air flows over the plane at low speeds. Of course, that's clearly not a factor in this case.

Anyway, herewith, the "groundspeed calibration table" for my MS, compared against a Garmin portable aviation GPS:

Tesla	Garmin
30	29.5
35	34.6
40	39.5
50	49.2
55	54
60	59.1
65	64.5
70	68.8
75	73.3

Tesla speeds were all held by the Cruise Control for several seconds on flat road. I don't know whether the Cruise Control keeps the precise speed you were going when you enabled it, or rounds to the nearest MPH and holds that. I also don't know whether the 5MPH bump on speed goes to the integral MPH or adds 5MPH to the precise previously-selected speed. I DO have high confidence that the Garmin speeds are accurate to within 0.25MPH, based on watching the fix drift when it's stationary.