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.

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