Optimistic Speedometer
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Optimistic Speedometer
According to Waze, GPS radar detector and town speed signs my speedometer is optimistic.
60mph on speedometer = 58mph
70mph = 67mph
80mph = 76mph
My Ford Bronco Sport display actual speed.
Any reason Porsche can't display the accurate speed?
60mph on speedometer = 58mph
70mph = 67mph
80mph = 76mph
My Ford Bronco Sport display actual speed.
Any reason Porsche can't display the accurate speed?
Rennlist Member
Joined: Apr 2020
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From: Ohio
I actually did a long trip for work this week (over 300 miles each way). On the drive up to my first leg, I noticed the exact same thing. On the drive back, I also used another GPS tracker on my phone to see if what Waze was saying was right. I also was 2-4mph under what the speedo was saying - so your numbers are exactly what mine were saying. Now I do have my winters on which are 19/20s and not the 20/21s but I think they have the same rolling circumference so that should not be it. Seems like the speedo is off a bit for sure.
On the plus side - I averaged 31.4 MPG on the trip, which is better than any car I own by a long shot. Much of it was highway going 75-80mph (speedo wise), but did have quite a bit of city as well, as I drove from Cincy to Chicago and was downtown for a couple of days.
On the plus side - I averaged 31.4 MPG on the trip, which is better than any car I own by a long shot. Much of it was highway going 75-80mph (speedo wise), but did have quite a bit of city as well, as I drove from Cincy to Chicago and was downtown for a couple of days.
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Joined: Feb 2009
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From: Newport Beach, CA and Melbourne, Australia
I guess margin for error to counter things like tire wear, different sizes etc.. In some jurisdictions the tolerance for speeding is as low as 2 mph, above that you get a ticket, so over reading by a couple of mph or km/h isn't such a bad thing.
Burning Brakes
Joined: Jul 2002
Posts: 904
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From: Smoky Mtns, Tennessee
My 2020 992 speedo is consistently 2mph too high (i.e., reads 82 = 80). I have 20" / 21" standard tires. All of the Porsche's my wife and I have owned have read high by 2-4 mph. My 2004 996 TT was always 4mph high. When I complained to SA early on (2003) they said that was normal / typical for Porsche's and history has seemed to bear that out.
The ONLY real issue I had was I once found (and I haven't checked this yet on the 992) is the odometer was also a little too quick, as in it showed a mile before it really was (I measured it using the old road, measured mile at the time on a particular piece of highway). This paid off later in the life of that particular Porsche when I had a part fail just outside of the warranty (according to the odometer), and I got it covered since I could easily show that it really should have been mathematically.
The ONLY real issue I had was I once found (and I haven't checked this yet on the 992) is the odometer was also a little too quick, as in it showed a mile before it really was (I measured it using the old road, measured mile at the time on a particular piece of highway). This paid off later in the life of that particular Porsche when I had a part fail just outside of the warranty (according to the odometer), and I got it covered since I could easily show that it really should have been mathematically.
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From: Southeast US
Most speedometers read 2-4% higher than actual for that very reason.
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From: Central Florida
My BWM did that and it was super annoying! Plus, it adds mileage to your vehicle that you didn't really travel. I guess it is one way to "improve" fuel mileage. What's next? A fuel gauge that tricks you into adding fuel when you don't need it? And temperature gauges faking you out? I think drivers of high end sports cars deserve accurate gauges. If get a ticket because I'm speeding, so be it!
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Instructor
Joined: Mar 2019
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Working in the industry I can tell you it is largely outdated technology BS. There are still big fines for under calling the vehicle speed. However, most vehicles have GPS now even if a navigation system is not enabled. This means that the speedometer can self calibrate from the GPS at any time and display actual speeds. The industry just continues to not do this for whatever reasons.
Rennlist Member
Joined: Aug 2020
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From: Orange County California
There is a company "Healtech" that makes a frequency converter for motorcycles called the "SpeedoHealer". It plugs into the wiring between the Vehicle Speed Sensor (VSS) and the computer that reads the VSS for the Speedometer. You can easily and accurately calibrate your speedometer. I have one on my Honda Goldwing and it works like a charm. Hopefully someone comes up with a similar product for our cars at some point. Here's a link to the Healtech website for anyone who might be interested: https://www.healtech-electronics.com/products/sh/
Drifting
Joined: Jun 2014
Posts: 3,095
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From: some where nice
My BWM did that and it was super annoying! Plus, it adds mileage to your vehicle that you didn't really travel. I guess it is one way to "improve" fuel mileage. What's next? A fuel gauge that tricks you into adding fuel when you don't need it? And temperature gauges faking you out? I think drivers of high end sports cars deserve accurate gauges. If get a ticket because I'm speeding, so be it!
where the fuel gauge drops slower at the top end as compared to the lower end , to give customers the false feeling that their cars does good on gas mileage
Intermediate
Joined: Apr 2012
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Something about the four satellites traveling at 7,000 mph at an altitude of 22,000 miles above earth vs the speedometer in the car that leaves me in awe of modern technology.
Of course the poor speedometer is affected by tire pressure, tire temperatures, summer vs winter tires, etc..
The GPS is not 100% accurate either but the government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.
Of course the poor speedometer is affected by tire pressure, tire temperatures, summer vs winter tires, etc..
The GPS is not 100% accurate either but the government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.
