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Car and Driver did an article on this, linked below. This is the pertinent paragraph:
"The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s."
I was out today and had the time to do some testing using a SkyPro XGPS160 receiver to check the speedometer accuracy against a high-accuracy GPS unit. It's not military accuracy but it's very good.
During the tests, the unit was reporting Excellent Horizontal accuracy with 29 satellites being tracked and an excellent signal-to-noise ratio. It was tracking a combination of GPS and GLONASS (Russian) satellites at the time as shown in one of the photos below. My tires are Michelin Pilot 4S's with a few hundred miles of wear. The road was either a wide-open high-speed Texas toll road or an access road next to it.
Samples were taken at a number of speeds. At each speed, the cruise control was set and when the car was on level road with the speedo reporting the set speed I took a few screenshots for each speed. If there was a little slope and/or it wandered above or below the set speed I didn't take the screenshot. Here are the results:
Car reporting 55 - GPS Average 55 (54.89, 55.01, 54.55)
Car reporting 60 - GPS Average 60 (60.19, 60.19, 60.07)
Car reporting 65 - GPS Average 65 (64.90, 65.36, 64.67, 65.25, 64.56)
Car reporting 70 - GPS Average 69 (69.74, 69.05, 69.85, 69.39, 69.16)
Car reporting 75 - GPS Average 75 (75.03, 75.49, 74.46, 74.69, 74.69)
Car reporting 80 - GPS Average 79 (79.40, 79.17, 79.06)
Here are a few of the speed and quality-related screenshots:
Checked mine today. Indicates 2 mph faster than actual ground speed per a radar unit.
StormRune is very fortunate. Most of the rest of us do not have such accurate speedometers.
The German law referenced in Car and Driver is also the excuse that BMW provides for its ****-poor speedometers. But we're in America and there's no reason our cars should be compromised by some dumb German law.
What I'd like to see is the ability to go into settings and calibrate my speedometer. Not only could I make it accurate, but it would allow compensation for tire wear or alternate wheels and tires of different diameters.
This would not be difficult to do. On a BMW I owned, it was possible to calibrate the trip computer mileage reading through the service menu. Sadly, that car didn't allow speedometer calibration either.
StormRune is very fortunate. Most of the rest of us do not have such accurate speedometers....
Yeah, I've been wondering about why I'm getting such close readings, but my Passport Max and Waze also tend to report the same or within 1mph of the speedo. I've tried to find an explanation... things like the Pilot 4S tires having a larger circumference maybe due to their lack of an N0 rating, but all of the diameters of the tires I sampled under the specs on Tire Rack are very close or the same and not enough to explain it. Even with a ĵ inch tread wear consideration of others possibly having worn tires vs my fairly new tires leaves me with a calculated difference maxing out short of 1% error. Maybe AWD models tend to be more accurate for some reason??? I don't know. It doesn't make sense to me that there would be a car-to-car variation outside of the mounted tire choices.
One downside to this inaccuracy, if indeed it exists in your car -- say your speedo is off by 3% -- the mileage on your car will always be 3% higher than actual miles travelled.
As has been noted, the two - in electronic systems - aren't connected. Odometers have to be accuraate.
Originally Posted by StormRune
I was out today and had the time to do some testing using a SkyPro XGPS160 receiver to check the speedometer accuracy against a high-accuracy GPS unit. It's not military accuracy but it's very good.
Damn, that's probably about as accurate as you can get. Since there's no longer a restriction on the data, it's probably as accurate as most any GPS, even military stuff!
Originally Posted by Valvefloat991
What I'd like to see is the ability to go into settings and calibrate my speedometer. Not only could I make it accurate, but it would allow compensation for tire wear or alternate wheels and tires of different diameters.
Some code readers with edit capability can do that. I have a Durametric for my Ram and can change the tire size manually and adjust it in 1 RPM increments to make sure it's accurate.
Yeah, I've been wondering about why I'm getting such close readings, but my Passport Max and Waze also tend to report the same or within 1mph of the speedo. I've tried to find an explanation... things like the Pilot 4S tires having a larger circumference maybe due to their lack of an N0 rating, but all of the diameters of the tires I sampled under the specs on Tire Rack are very close or the same and not enough to explain it. Even with a ĵ inch tread wear consideration of others possibly having worn tires vs my fairly new tires leaves me with a calculated difference maxing out short of 1% error. Maybe AWD models tend to be more accurate for some reason??? I don't know. It doesn't make sense to me that there would be a car-to-car variation outside of the mounted tire choices.
You may be right about the AWD cars being more accurate. As posted earlier, my 2017 C4GTS speedo matched the GPS readout on my phone. I have the stock tires with about 1,500 miles.
I have no idea why Porsche would choose to make the AWD speedo accurate while making the RWD speedo read high. Definitely a mystery.
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