Speedometer Accuracy?

[Rbwalters]

I think my speedometer might be off by a couple MPH in my MY25 Turbo S ( 21 inch wheels). I do have different wheels/tires than came from the factory, but they are the same size.

I've noticed roadside speed radar displays seem to consistently under-read my speed, compared to my car's speedometer by 2-3 MPH, depending on speed. Meaning, when my car speedometer says I'm going 70, the roadside display says 67. Certainly, the displays could be inaccurate, but it seems to be consistent between different ones. There is one close to my house, and I've passed it a few times while on cruise control, and when there were no other cars, with consistently the same result.

I'm concerned that if my car is over reading my speed, the odometer may also be incorrectly adding miles.

Anyone had any experience with this?

There is one of those 1 mile speed checks on Interstate-5, but I haven't been that direction in a while with the car. Other than that, is there some other way to check the accuracy of the speedometer? I'd rather avoid going to the dealer.

Thanks.

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[dbroker]

Hi - I have experienced this exact same thing both of my Taycans as well. I have not explored it any further.

 

[SergeyIndy]

This is a known issue. Speedometer displays +2mph more than actual and could be as high as +3mph. @daveo4EV is an expert on this topic. So I set my cruise to say 72 mph if I want actual to be 70 mph.

 

[Fish Fingers]

+2-3 mph is by design on all cars I read somewhere previously.

It has certainly been the case on all my recent cars.

So I set cruise to 2 mph over the limit when navigating our (many) SPECS speed camera zones.

 

[Jasper4S]

Indeed, that is by design. Legally a speedometer is not allowed to under-read, in Europe this is defined by ECE Regulation 39. Manufacturers also add a margin to account for differences in tires such as wear, brand, pressure and type.

That also explains why we can all drive “faster” than the advertised top speed. My car is listed at 250 km/h, but I managed to reach 263 km/h before it stopped accelerating. Most likely this is due to the built-in margin, since my GPS speed would have been the actual 250 km/h.

 

[whitex]

What others have said, cars tend to err on the high side. However, I should also say roadsite radar signs are going to typically show you slower than your actual speed, as they are not calibrated for the cosine angle error. This is also why as you get close to the sign (and there are no cars behind you) even at constant speed the sign will start showing lower numbers.

Try comparing against GPS speed (e.g. using your smartphone) in good weather on a straight road, going steady speed.

 

[whitex]

Since nobody mentioned it so far, there is another angle to this. If the error is always 2mph, does this this potentially lower the car's warranty coverage, since the odometer usually just integrates the speedometer?

 

[Jasper4S]

Since nobody mentioned it so far, there is another angle to this. If the error is always 2mph, does this this potentially lower the car's warranty coverage, since the odometer usually just integrates the speedometer?

No, this normally has no impact on your warranty.

The speedometer shows speed + margin. Odometer is correct (also calculated, but more “correct” than the speedometer). So no impact on warranty or leasing etc

And does it really matter? It is al relative.

 

[whitex]

No, this normally has no impact on your warranty.

The speedometer shows speed + margin. Odometer is correct (also calculated, but more “correct” than the speedometer). So no impact on warranty or leasing etc

If you are right, you can calculate how much faster your speedometer is by simply driving at fixed speed (cruise control) for a few miles and timing it - your true speed (again, if you trust the odometer) would be the distance, divided by time (in hours).

And does it really matter? It is al relative.

My Taycan average lifetime speed is 27mph (according to MyPorsche). A 2 mile over-estimate would mean I lose a few thousand miles from my 60k miles warranty.

 

[f1eng]

Every car I have ever owned under reads for the reasons already mentioned in this thread.
I "calibrate" the speedo at the various legal limits using a GPS speed app so I know the margin.
FWIW the Taycan is closer to "correct" than any other car I have checked, 5 to 10% is normal.

 

[Shug]

I always use Waze when driving away on a journey, this confirms all that has been said in this thread.

I set my cruise control against the Waze speed, not the vehicle's indicated speed.

I can confirm that 77MPH indicated is 74MPH Waze / GPS speed.

 

[abatis]

I have this issue with my Cayenne. However, my 2021 CT is spot on with radar signs and GPS speed with 20-inch wheels.

 

[Tyros]

I first noticed the speed discrepancy years ago when we bought our first Cayenne. I checked with the dealership, and they explained that this is normal and expected. Over the years, I’ve driven many Porsches and found the behavior to be consistent. The difference also varies with speed: at around 25 mph it’s about 2 mph, at 60 mph it’s about 3 mph, and at 90 mph it increases to roughly 4 mph. So, when I observed the same behavior in our Taycan, I wasn’t surprised. I now factor that in my driving routine.

 

[Blackbird]

Porsche speedometer reading 2 mile per hour above the actual speed goes back at least two decades. My previously owned 2003 C4S and 2007 Cayman and current 2023 Taycan all read 2 mph over.

 

[Red4S]

I do not believe the increase in speed is not showing up in increased mileage. It will definitely reduce your warranty coverage. Confirm your speed with GPS and check your mileage with interstate mileage markers. Check distance and speed with GPS.

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