Speedometer off 3-4 mph at 70-80mph

[Fleming23]

I have not reached out to my dealer yet, but had my cruise control set to 80 mph and though it odd traffic was all over my rear bumper. Pulled up Waze and realized my actual speed is around 4 mph slower than indicated at 80. Even at lower speeds (50 +/-) my speedometer is reading 2-3 mph faster than actual GPS speed. I am on 22" Spyder wheels, although not sure why that shoudl matter. I need to verify the odometer is reading correctly.

Anyone else noticed this? I would expect 1-2 mph, but 3-4+ is excessive, and quite annoying.

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[daveo4EV]

this is common and an offset the germans put in all their speedo's - knowing this I adjust my Cruise Control set speed appropriately - all my Porsche's have this "offset"

I'm reminded of this "offset" every time I drive past a "you are going this fast" radar sign in various places on the highways in my local area - I'm able to note a 2-4 mph "adjustment" from what the external radar shows and the indicated speed on the dash inside the vehicle…

73 mph indicated speed on the dash of my Macan EV seems to yield 70 mph on any external measurement device…

this is similar in my 2021 Cayenne, 2022 911, 2023 911 and has been consistent with previoius year models of Porsche's I've own in the past.s

it's not a "defect" it's by design and the way Porsche sets up their vehicles and speedometer for what ever reason.

 

[rmwrip]

I have owned several BMWs. All under-reported the correct speed. I just set cruise 3 mph over what I want it at.

 

[alvaro]

I dunno, Id trust the car before I trust Waze. Cell latency and all..

 

[daveo4EV]

I dunno, Id trust the car before I trust Waze. Cell latency and all..

I agree with trusting car before waze, but it's a fact that Porsche speedometers underreport your speed _ALL_ known external speed meters I've ever used/seen - so the car is reporting your actual speed + 2 mph…

 

[SergeyIndy]

I just tried to compare the Speedometer reading in my Taycan vs. Google Maps Speedometer and sure enough the difference is exactly 3 mph at speeds between 30 to 70 mph with Taycan +3 mph vs. Google Maps value. Now if someone can measure using some calibrated equipment to tell us which one is right ! I will now be looking at the speed measuring pods to see what they say as the other source of reference.

 

[daveo4EV]

2 mph has been the measured and indicated "delta" for the following porsche's I've owned when compared to external speed camera's/radars

2011 911 GT3
2012 911 GTS
2008 911 4S
2015 Boxster X
2015 911 GT3
2018 911 GT3
2010 911 GT3 Cupcar
2015 911 GT3 Cupcar
2020 Taycan Turbo
2021 Cayenne TurboS eHybrid
2022 911 TurboS Cab
2023 911 GT3 RS
2023 911 GT3
2024 Macan Turbo EV

if the Porsche speedometer indicates 70 mph on the dash - external actual physical speed is 68 mph…

this has also been true when I've rented/used Porsche's in Europe

 

[daveo4EV]

this has also been discussed extensively on rennlist for years - +2 mph is the indicated speed delta in most/all modern Porsche vehicles measure from a majority of "true" speed indicators…

+2 mph indicated speed is also the delta different between the Porsche speedometer and GPS based track telemetry equipment I've had installed in various 911 track cars…VBOX, AIM, and APEX pro devices - all high resolution GPS telemetry tools/devices I've used show a +2 mph delta indicated by Porsche speedometer vs. their "true gps speed" indication…70 mph on the porsche dash board equates to 68 mph on the high-resolution GPS speed indication…

final straw on this camel's back is even Porsche's own Track precision app shows true GPS speed (ironically how ever using teh vehicle's GPS data over the wifi connection) and the Porsche track precision app will show "true" GPS speed when the vehicle reports +2 mph…so even Porsche's own software shows the "true" speed using GPS location details where the vehicle's speedometer is +2 mph over GPS indicated speed.

I've never found any independant speed indicators that agreed with the Porsche dash board speed - even other cars side by side with cruise control set and talking to a friend over cell phone…

 

[Fleming23]

I would be okay with 2 mph, but I am seeing 3-4. Not just using Waze, but it was a combination of speed box on my phone as well as a school zone speed sign I passed this morning which seemed to reasonably agree with phone gps.

I guess I’ll just get used to it, but a bit annoying to report so far off.

 

[daveo4EV]

I would be okay with 2 mph, but I am seeing 3-4. Not just using Waze, but it was a combination of speed box on my phone as well as a school zone speed sign I passed this morning which seemed to reasonably agree with phone gps.

I guess I’ll just get used to it, but a bit annoying to report so far off.

3-4 mph can also be tire size…any phone GPS I wouldn't trust inside the car because it's sample rate is pretty slow - the school speed sign is more compelling vs. any phone based app…

2 mph is the "base" delta - any other differences would be tire diameter/wheel size…in any case it "over-reports" - so you can bump your cruise control up a bit and know you're not getting a ticket.

 

[henrus]

It’s not just Porsches. Most cars under report.

 

[wmras]

2 mph has been the measured and indicated "delta" for the following porsche's I've owned when compared to external speed camera's/radars

2011 911 GT3
2012 911 GTS
2008 911 4S
2015 Boxster X
2015 911 GT3
2018 911 GT3
2010 911 GT3 Cupcar
2015 911 GT3 Cupcar
2020 Taycan Turbo
2021 Cayenne TurboS eHybrid
2022 911 TurboS Cab
2023 911 GT3 RS
2023 911 GT3
2024 Macan Turbo EV

if the Porsche speedometer indicates 70 mph on the dash - external actual physical speed is 68 mph…

this has also been true when I've rented/used Porsche's in Europe

We have only purchased 5 new Porsches, but all (except the 2016 GT4) are exactly the same: 2 mph over actual above 20 mph. You can see the odd speedometer behavior around 20 mph. The 2016 GT4 was exactly correct at all highway speeds (slight error at the 100-mph cruise control setting).

 

[PrudentOcean]

It’s not just Porsches. Most cars under report.

Did you mean over report?

 

[Wivenhoe]

I posted this in another thread -

In Europe and the UK the law states. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So at say 70 mph, it can never be less but could be so inaccurate it could comply by reading 7 + 6.25 + 70=83.25 !

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.

I’m not defending it, but I can’t see any German manufacturers recalibrating speedos for different countries.

 

[PrudentOcean]

I’m not defending it, but I can’t see any German manufacturers recalibrating speedos for different countries.

They would need to be in compliance with the local laws and regulations. Over-reporting would be OK unless prohibited, and that doesn't seem to be the case anywhere.

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