Speedometer accuracy

[Gary Gerrard]

I have noticed my speedometer reads 2 mph higher than all other readings. In other words, I am going 2 mph slower than indicated. Anyone else have this experience.

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[Midlifecrisis]

I have noticed my speedometer reads 2 mph higher than all other readings. In other words, I am going 2 mph slower than indicated. Anyone else have this experience.

I think this is normal. The difference gets greater as your go faster. It reduces the chance of inadvertently breaking the speed limit

 

[dtich]

Yes, in my experience most performance vehicles err on this side.

 

[bn8959]

Also needs to allow for the slight change in circumference of the wheels as the tyre tread wears down. So you may find it more accurate with new tyres and it gets further out as they wear.

 

[WattTurbo]

Mine is around 2.5 mph optimistic. My 911 is 99% accurate so it’s a Porsche thing but a Taycan thing

 

[tchavei]

Usually the law allows small upward deviations but it's illegal for a speedometer to show below the actual speed. Hence why manufacturers rather err by excess than going below.

 

[DougFrisk]

I have noticed my speedometer reads 2 mph higher than all other readings. In other words, I am going 2 mph slower than indicated. Anyone else have this experience.

The law in the US is that the speedometer in the car may read up to 5% higher than the actual vehicle speed. It may not read lower than the actual vehicle speed ever. For that reason most manufacturers selling vehicles in the US aim for a speedometer that reads a couple of percent fast.

I couldn't tell you where I read that bit of trivia many years ago, but since the advent of phones with GPS I've found it to be about right.

 

[mjw930]

Porsche sedans and SUV’s, as well as all other cars in the VW group routinely read 5% high in the US. Their sports cars, which use platforms and technology not shared anywhere else in the VW group, usually are pretty much spot on. This has been the case for over 20 years.

So, a company which didn’t think twice to create software that hacks the EPA emissions test is worried about an obscure regulation regarding speedometer accuracy? I don’t buy it, in my mind it’s lazy engineering.

 

[Sidicks]

Porsche sedans and SUV’s, as well as all other cars in the VW group routinely read 5% high in the US. Their sports cars, which use platforms and technology not shared anywhere else in the VW group, usually are pretty much spot on. This has been the case for over 20 years.

So, a company which didn’t think twice to create software that hacks the EPA emissions test is worried about an obscure regulation regarding speedometer accuracy? I don’t buy it, in my mind it’s lazy engineering.

I’m afraid that your mind is wrong!

As explained above, speedometers are legally not allowed to read lower than the actual speed, so are engineered to read slightly above.
Also note that actual reading will depend on tyre wear, with the difference between a brand new tyre and a fully worn tyre being around 3%.

 

[mjw930]

I’m afraid that your mind is wrong!

As explained above, speedometers are legally not allowed to read lower than the actual speed, so are engineered to read slightly above.
Also note that actual reading will depend on tyre wear, with the difference between a brand new tyre and a fully worn tyre being around 3%.

No, the difference between a new and fully worn tire is not 3%, it’s much closer to 1%. For a Taycan rear tire, the new diameter is 29”. 3% is .87”. Useful tread depth is 6/32nds. 12/32nds is .375” which is 1.3% as a worst case. Most manufactures engineer to a 1% tolerance.

 

[Sidicks]

No, the difference between a new and fully worn tire is not 3%, it’s much closer to 1%. For a Taycan rear tire, the new diameter is 29”. 3% is .87”. Useful tread depth is 6/32nds. 12/32nds is .375” which is 1.3% as a worst case. Most manufactures engineer to a 1% tolerance.

I lazily did a quick google search and didn’t check the numbers.
I think their calculation rounded up and used small (14”) wheels.
Your figures make sense.

 

[RAHRCR]

Mine shows 3 mph over actual. Dealer says this is “normal”……I say ???

 

[Jonathan S.]

^ That is definitely normal for our BMW i4 M50. Ditto for many other BMW models going by the posts on the equivalent forum for the i4. But for our Taycan (2022 4 CT), I'm measuring a discrepancy of at most 1 mph in the low 70s. This is based on setting the cruise control while driving on relatively straight and level interstate stretches, then periodically (and briefly) glancing at a Garmin fenix 5X (which although not made for this purpose, has been consistent with i4 posters who used a professional-level GPSr).

Note that the U.S. legal spec is surprisingly broad:
https://www.ecfr.gov/current/title-...ubchapter-B/part-393/subpart-G/section-393.82

 

[Uknown]

I have noticed my speedometer reads 2 mph higher than all other readings. In other words, I am going 2 mph slower than indicated. Anyone else have this experience.

Same!

sadly the other day the road sign on dash wasn’t accurate and I got my first ticket in maybe 20 years… mind you I would have likely still gotten a ticket even if it had been accurate ?… but apparently I was REALLY speeding what i thought was 5 over, was really 15 over.

 

[tchavei]

When I swapped my 19'' with 21'' wheels (both with factory recommended tire dimensions) and had both standing side by side, it was clearly noticeable that the 19'' wheel + tire combo sat HIGHER than the 21'' combo aka the 19'' wheel configuration had a bigger circumference when compared to the 21''.

I assumed that choosing the "21'' wheel option" in the PCM would calibrate/match the speed reading to the new combo but no. It reads exactly 5km/h higher at 120km/h then what GPS says which makes sense since the circumference is now smaller making the wheel turn more for any given speed then on 19''.

Cheers
Tony

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