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Speedometer mph vs gps mph
- Thread starter laceyboy
- Start date
All depends on your make and model of GPS unit... 
But referring to MY, then no. Are you using OEM tires or did you install something else? Diameter is supposed to 28.0 for MY, that is what the speedo is expecting. If your tires are a slightly bigger diameter, either because you are using something different, or because they are over inflated, then that could cause it. Also, how do you know that your GPS measurement is accurate? Just because it is GPS does not mean precise. Number of satellites in view, the accuracy error, etc. all factor in. GPS could vary depending on bunch of factors.
But referring to MY, then no. Are you using OEM tires or did you install something else? Diameter is supposed to 28.0 for MY, that is what the speedo is expecting. If your tires are a slightly bigger diameter, either because you are using something different, or because they are over inflated, then that could cause it. Also, how do you know that your GPS measurement is accurate? Just because it is GPS does not mean precise. Number of satellites in view, the accuracy error, etc. all factor in. GPS could vary depending on bunch of factors.
73Bruin
Member
I would assume that almost anything on the MY could be addressed by software updates. However, I would be very surprised if Tesla would ever base the speedometer on the GPS on a real time basis, or even use it to correct its errors periodically. From what I have read in the past, speedometers are more a quide than an absolute reference and for reasons tied to commercial vehicle rules read higher than the actual speed just like you are experiencing. There is a great Quora write up about this.
Is it true that car speedometers are slightly inaccurate to reduce speeding? - Quora
I forgot to provide a bit more info.All depends on your make and model of GPS unit...
But referring to MY, then no. Are you using OEM tires or did you install something else? Diameter is supposed to 28.0 for MY, that is what the speedo is expecting. If your tires are a slightly bigger diameter, either because you are using something different, or because they are over inflated, then that could cause it. Also, how do you know that your GPS measurement is accurate? Just because it is GPS does not mean precise. Number of satellites in view, the accuracy error, etc. all factor in. GPS could vary depending on bunch of factors.
My MY came with OEM parts, no aftermarket.
GPS I'm talking about here is my Galaxy S21 Ultra and using Google maps for navigation.
Mph reported on my phone matches with my previous vehicles I owned.
I will check with my MX later.
I stumbled upon this thread because I had the same exact concern. My new model Y (all original parts) shows 2-3 MPH more than the GPS in my radar detector. I don't expect it to be perfect, but it's a bigger discrepancy than I would expect. My Mazda actually reads slightly less than the GPS, but it is very close to accurate. When I drive by a speed measuring sign on the side of the road, the GPS always exactly matches that reading, while the MY shows at least 2MPH and sometimes 3MPH higher. That's a pretty substantial difference.
Driving yesterday I passed 2 separate CHP speed warning signs that show your speed as you pass when there's a temporary lower speed limit. Those are calibrated often. Both of them showed the exact speed my Model Y was at, even during deceleration. I haven't compared to my GPS, but it made me confident that the speed is calibrated correctly.
Tha_Ape
Active Member
trust the GPS *especially in a straight stretch of road). Speedometers are optimistic in general. I find them to be about 2-3mph off in the slower direction.
the wheel speed sensors are usually better. I have a Comma.Ai in my Prius Prime which goes off the front left wheel speed sensor and its about 2-3moh off the Speedometer (as is GPS). I think manufacturers play it safe and give you an "optimistic" view of your speed. likely saves them a headache with lawsuits
the wheel speed sensors are usually better. I have a Comma.Ai in my Prius Prime which goes off the front left wheel speed sensor and its about 2-3moh off the Speedometer (as is GPS). I think manufacturers play it safe and give you an "optimistic" view of your speed. likely saves them a headache with lawsuits
Interesting thread. My Miata (now sold) and my Bolt (keeping forever) both read dead nuts on with the GPS in my phone... If my MY LR shows higher speedometer reading than GPS it will encourage me to get larger diameter tires to make the speedometer match reality If the speedometer is off by a percentage it explains why it is sometimes off by 2 mph and other time it is off by 3 mph... only off by 2 mph at slower speeds, 3 mph at higher speeds. In order to compensate for a 2 mph discrepancy at 60 mph you actually need a full inch larger diameter (half inch taller sidewall). A 255/50/19 tire would be perfect if this is a real world discrepancy not a figment of instrumentation.
Keith
If the speedometer is off by a percentage it explains why it is sometimes off by 2 mph and other time it is off by 3 mph... only off by 2 mph at slower speeds, 3 mph at higher speeds. In order to compensate for a 2 mph discrepancy at 60 mph you actually need a full inch larger diameter (half inch taller sidewall). A 255/50/19 tire would be perfect if this is a real world discrepancy not a figment of instrumentation.Keith
Midnightsun
Active Member
In general I would say 99% of all cars out there read 1 or 2 or even more mph faster than actual. I did play with tire size on a previous car and got it dead on which anyone can do HOWEVER!!!!.
On a previous vehicle my reading was about 2kph faster than actual which was close enough, I put on winter tires, same brand, same size. I now was reading 5kph faster. Called my good friend who sold me the tires and owns a tire shop and told him something must be wrong. He called the distributor and was told this is normal as the overall diameter has a tolerance which is within spec. Bottom line is a 28" diameter tire may be slightly smaller or bigger. Try comparing speeds next winter when you swap and I can almost guarantee you there will be a difference in indicated speedo speed.
A new tire and completely worn out tire will also differ as the diameter will obviously have changed.
I ordered my Tesla with the 20" wheels, in my case it reads pretty good showing about 1kph faster than actual. My winter tires were about 2kph off although on 19" wheels yet the spec showed the same diameter as the OEM 20".
On a previous vehicle my reading was about 2kph faster than actual which was close enough, I put on winter tires, same brand, same size. I now was reading 5kph faster. Called my good friend who sold me the tires and owns a tire shop and told him something must be wrong. He called the distributor and was told this is normal as the overall diameter has a tolerance which is within spec. Bottom line is a 28" diameter tire may be slightly smaller or bigger. Try comparing speeds next winter when you swap and I can almost guarantee you there will be a difference in indicated speedo speed.
A new tire and completely worn out tire will also differ as the diameter will obviously have changed.
I ordered my Tesla with the 20" wheels, in my case it reads pretty good showing about 1kph faster than actual. My winter tires were about 2kph off although on 19" wheels yet the spec showed the same diameter as the OEM 20".
Because Tesla has set the AP/NOA (and FSDb) top speed to 80mph, this is no longer just a nag. This is more pronounced in the West where 80mph is a predominant speed limit on freeways. It has become a hazard, and I am the slow car complicating the flow.
For my 2019 M3, the UI displays 80, the canbus displays 79, and the GPS show 78. I think a few failures are operating to create this situation: improper calculation and conversion to the UI, improper measurement of speed across the ground, California bias on acceptable maximum speed limit.
For my 2019 M3, the UI displays 80, the canbus displays 79, and the GPS show 78. I think a few failures are operating to create this situation: improper calculation and conversion to the UI, improper measurement of speed across the ground, California bias on acceptable maximum speed limit.
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Here's the odd thing for me. I posted a message in this thread about the discrepancy that I found in my MYLR when I first got it six months ago. But driving it over the holidays, I passed quite a few of these measuring devices and found that the Tesla's speedometer is now quite accurate. One of the devices is near my home, so it is one of the same ones that I was looking at in June. I really can't explain it. It's not likely to be tire wear because the car has only 4000 miles on it. Tire pressure? Temperature? Some form of self-calibration? I just don't know. When I looked at this question in June, the GPS in my radar detector agreed with the devices on the road - only the Tesla was wrong. Now the meters and the radar detector GPS are still in agreement, but the Tesla now also agrees. Weird. At least it's good news.
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