Have a police officer sit on a side street with radar. Make numerous passes of said officer at ever increasing speeds. He will eventually let you know when you have exceeded the speed limit.
-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
Speedometer Accuracy?
- Thread starter ratsbew
- Start date
I thought I read that the odometer is more accurate. (They aren't 100% linked like in a mechanical speedometer/odometer.)
Now with the stop sign and traffic light beta, where it is not possible to go over the speed limit with autosteer, this can have people behind you grumpy. I am not using autosteer for now so I can bump the speed up to going the speed limit. My car has been off by 1MPH since purchased. All permanent traffic radar signs read 1 mile slower than the car as does my phone.
Speedometers will always read high. The laws as much as require it. (They are legally not allowed to under-report your speed, so rather than try to be exact they over report it.)
If I were Tesla I would use the GPS to calibrate the speedometer. My Garmin GPS for my bike does this to calibrate the wheel magnet. The Garmin uses the wheel magnet as the primary, but it collects data over time (on straightaways) to make sure it's accurate.
JCtx
Member
More than a percentage, every time I pass over a street radar, I've been 2 mph higher than what radar says, whether it's 30+, 40+, or 70+. My car is new, so as the tires wear (slightly less circumference), speedo should become more accurate, but maybe never 100%. We'll see 
.
.
Candleflame
Active Member
The speedo.isnt allowed to overread. For most cars there is a tolerance up to 10% below actual speed i.e. a 2000 bmw will usuaally read around 104kmh when its going 100.
Teslas are quite accurate. I find they are only off by 1kmh when the tires have 3.1 bar cold in them. So thats like 0.4mph.
Teslas are quite accurate. I find they are only off by 1kmh when the tires have 3.1 bar cold in them. So thats like 0.4mph.
While the speedometer as a device may be accurate, I’m curious to know what TMC members have done when installing wheels and tires where the changes in diameter affect accuracy beyond what is allowed.
Candleflame
Active Member
Well noone here says it overreads? They all say it underreads which... Tbh 1 to 1.5mph is easily possible if you only have 39psi in the tires...
GPS dynamic speed accuracy varies depending on several factors including antenna type and location. UBLOX (common automotive GPS chip maker although I don't know if Tesla uses them) indicates real-world dynamic speed accuracy 68% of the time between 0.1m/s and 0.8m/s which is 0.22mph to 1.79mph.
I don't know but seems logical because of less than ideal antenna placement due to being inside the car, your phone might be worse.
So the point is to keep system inaccuracies in mind when making comparisons to a reference speed.
No sleep lost over this for me
Shelburne
Member
I wouldn't trust your phone's GPS as compared to your Tesla's speedometer. I use a ski app on my iPhone that, among other things, clocks my speed. Curious about it's accuracy, I tested it against a friend's radar gun. He's a cop and the radar gun had been calibrated just days before we tried this. We did three comparison tests and each time the phone disagreed with the radar gun by between 3 and 6 mph. The radar gun said I was going slower than the app each time.
metroplex
Gearhead
The GPS on most phones are 1 Hz. You need to use a 10 Hz or faster GPS (which some are aviation grade). But with a 1 Hz GPS, you can do it as long as you have a flat section of road and be able to maintain that speed for several seconds. I've compared my 1 Hz to the 10 Hz GPS on such conditions and they will be almost dead-even. It will match the K-band radar displays they have on some roads (Your Speed Is...) On every Ford/GM made since 2005 or so, it is easy to calibrate the electronic speedometer for an accurate reading. Either with the tire circumference or rev/mile setting in the PCM tuning, or in Fords in the past 10 years, the Body Control Module tire circumference setting via FORScan. Tesla doesn't seem to have this capability for the end-user to adjust this for calibrating their speedometer, and my 22 Model 3 LR indicates 1 mph faster than actual/GPS at 70 mph. It's within "spec" but not accurate. Seeing how people have observed the same 1 mph discrepancy for the past 4 years tells me this is something "normal" with Tesla and they don't have any intention of fixing it.
Yes, it is on purpose, and most car makers do it. The reason is that they are legally not allowed to ever report a speed that is below what you are moving. So they opt to report a faster than actual speed. (Some manufacturers by up to 5 MPH at highway speed.)
NHTSA even complained about it when they saw videos showing the car reporting 1 MPH at a stop sign and the car continuing. (That FSD was not actually stopping.) Tesla had to provide video proof that even though the speedometer said 1 MPH, that the wheels were not moving, and that the reason for the 1 MPH report was to keep in compliance with the other NHTSA rules.
metroplex
Gearhead
My 2014 SHO and 2017 Fusion Sport were spot on from the factory. It's a matter of adjusting the tire circumference or rev/mile. Nothing to do with a base offset by 1 mph or some number. So it will read 0 mph at 0 mph. That's why when you calibrate your own speedometer with the help of GPS that you're gauging the speed delta (in mph) at different speeds and seeing the percentage difference. Then applying that to the tire circumference or rev/mile in the ECU or Body Control Module of the vehicle you're tuning. Super easy to do on a Ford or GM. I just had to adjust it in my 2017 Fusion after going with aftermarket tire sizes (235/45R19 instead of the stock rubber band 235/40R19).
The last vehicle I bought new that couldn't allow me to easily adjust the speedometer was my 2000 Ford Crown Victoria (mechanical speedometer gear) and now the 2022 Model 3 LR.
Similar threads
- Article
