Speedometer showing lower than actual speed
Thread Starter
Out Of Control!!
Joined: Nov 2018
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From: San Francisco Bay Area
2019 C63CS
Speedometer showing lower than actual speed
Has anybody else noticed their speedometer showing a lower speed than measured via GPS? Fully understand that speedometers are not a 100% accurate, but my understanding is that they are never allowed to show a lower than actual speed. They can show a higher speed, but not a lower. However, mine is showing lower. I've gotten a new radar detector recently and it shows GPS speed, which is quite a bit higher. At 70, I'm going more like 73, and at 85, I'm almost doing 90 in actuality which kinda worried me. That's 5% tolerance (acceptable), but in the wrong direction. I double-checked with my phone and it agrees with my radar detector. I do have PS4S instead of the stock PSS but same sizes, and I just checked the specs. The PS4S has the same diameter as the PSS for each of the sizes. The rears are actually slightly taller in both cases.
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2018 GT R; G82; M340
Has anybody else noticed their speedometer showing a lower speed than measured via GPS? Fully understand that speedometers are not a 100% accurate, but my understanding is that they are never allowed to show a lower than actual speed. They can show a higher speed, but not a lower. However, mine is showing lower. I've gotten a new radar detector recently and it shows GPS speed, which is quite a bit higher. At 70, I'm going more like 73, and at 85, I'm almost doing 90 in actuality which kinda worried me. That's 5% tolerance (acceptable), but in the wrong direction. I double-checked with my phone and it agrees with my radar detector. I do have PS4S instead of the stock PSS but same sizes, and I just checked the specs. The PS4S has the same diameter as the PSS for each of the sizes. The rears are actually slightly taller in both cases.
Thread Starter
Out Of Control!!
Joined: Nov 2018
Posts: 11,431
Likes: 5,340
From: San Francisco Bay Area
2019 C63CS
No, I haven't, but as said my phone showed the same GPS speed as the radar detector. It's a good idea, though. Might just check it with my wife's car to get another data point. Just trying to decide if I should bring this up during my next service visit and have them recalibrate it. Just not sure if that's gonna make it better or worse...haha. Has anybody ever recalibrated their speedometer?
Senior Member
Joined: Jan 2016
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From: ATL
2020 C63S Coupe, 2021 Bmw X5M Competition
Yep, same here. I had to be really careful driving through Virginia when I bought the car in Maryland. My wife kept telling me to slow down then I realized what was happening. My car is reading 2-3 mph lower and hers is 3-4 higher.
I've always seen cars reading higher than actual speed but never lower.
I've always seen cars reading higher than actual speed but never lower.
Thread Starter
Out Of Control!!
Joined: Nov 2018
Posts: 11,431
Likes: 5,340
From: San Francisco Bay Area
2019 C63CS
Thanks guys. After doing some research it turns out the USA is quite lax on regulating this. There appears to only be a federal standard for commercial vehicles and it allows for +-5mph accuracy at 50mph, so I guess technically reading a lower than actual speed is acceptable. It's mainly regulations in Europe that specifically state it can't read lower than actual, only higher than actual is allowed. Would have expected that they calibrated the speedometer to meet all these standards with one setup, but I guess not. Oh well, I'll probably won't bother and just keep it in mind moving forward.
Thread Starter
Out Of Control!!
Joined: Nov 2018
Posts: 11,431
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From: San Francisco Bay Area
2019 C63CS
What's interesting is that I bought this new radar detector after discovering last October that my old one wasn't working properly anymore and I got a ticket for doing 85 in a 65. I was in fact doing 85. Had DISTRONIC set to 85. Perhaps the officer deducted a tolerance, but it was in line with the speed I was going. The good news is the officer never properly filed the ticket with the court and the statute of limitation expired on Monday. They have 1 year here to file charges. Problem solved itself 
.
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Junior Member
Joined: Nov 2020
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From: North Carolina
C63 S AMG Cabriolet
Just stumbled upon this thread, and wanted to report that my speedometer also shows me going 1-2 mph slower than what GPS (Escort Max 360c) tells me. That surprised me, and is rather disappointing. Regardless of legality, a speedo should never under-report your speed, particularly on a car like this.
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From: Flat Earth
AMG C63s Sedan , AMG C43 , AMG G63 , GLK 250
Just stumbled upon this thread, and wanted to report that my speedometer also shows me going 1-2 mph slower than what GPS (Escort Max 360c) tells me. That surprised me, and is rather disappointing. Regardless of legality, a speedo should never under-report your speed, particularly on a car like this.
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From: Toronto, ON
2021 Mercedes C63s AMG Coupe
My speedometer reads 3km/h off at all speeds. Which is very weird. If there is an error, it should be a certain percentage and should read more and more as the car goes faster.
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From: DFW, Texas
2017 AMG C63
Given your evidence, is it an assumption that the radar / gps are more accurate than the car?
For another data point, drive by a roadside cop radar sign if you can find one, and see how close to your vehicle display it is - those are calibrated at least annually and are usually pretty accurate.
For another data point, drive by a roadside cop radar sign if you can find one, and see how close to your vehicle display it is - those are calibrated at least annually and are usually pretty accurate.
Thread Starter
Out Of Control!!
Joined: Nov 2018
Posts: 11,431
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From: San Francisco Bay Area
2019 C63CS
Given your evidence, is it an assumption that the radar / gps are more accurate than the car?
For another data point, drive by a roadside cop radar sign if you can find one, and see how close to your vehicle display it is - those are calibrated at least annually and are usually pretty accurate.
For another data point, drive by a roadside cop radar sign if you can find one, and see how close to your vehicle display it is - those are calibrated at least annually and are usually pretty accurate.
So if the GPS shows 10% higher than actual, but the car's speedometer only shows 5% higher, then the GPS would inevitably show a higher speed than the car's speedometer. GPS accuracy increases at higher speeds, and in my case the difference is definitely a percentage and not an absolute amount. It gets higher as I drive faster.
Your suggestion is a good one and based on empirical data it seems my car's speedometer is in line with the roadside radar signs, so I no longer consider this to be an actual issue. I think not considering initially that the GPS speed of my radar detector could simply deliberately show a higher speed than the speedometer in the car led to an incorrect conclusion.
