My 2003 S-Type V6 displays what is apparently the wrong speed, by around 5 MPH at 70. In other words, the speedometer shows 70 when I'm going 65. I believe I have the stock wheels and correct tire size. What could cause this?

I've checked it with a GPS and with another vehicle I was following while talking to the driver on my cell phone.

 

Interesting--my 2004 V8 does exactly the same.I just assume that it is a design feature to enhance fuel consumption and save me from speeding tickets!

 

Just about all cars have an 'optimistic' error. By law, they cannot under indicate and must either be precise or over-indicate by up to a few percent.

It's always been that way, we didn't have anything accurate like GPS to make reference to.

Mine indicated about 3km/hr fast with brand new tires, it's up to about 6km/hr now as they wear.

 

Haven't checked our '05 STR, but both of the Porsche 911s we own read 10% high. This is a known common issue with the 911s.
Many in that community send the speedometer gauge out to a company like North Hollywood Speedometer for recalibration.
Not sure what the possibility is with the S Types.

Cooper

 

Use ETM and see what it displays

 

Thanks! I searched for ETM since I had no idea what you were talking about and found all the info. Will do that this weekend, sounds like fun!

 

I'm betting that will be optimistic too.

 

On my 2005 STR the GPS and speedometer agree so close that I can't tell the difference.

How can the GPS be off and how would we check it?
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I think ETM's value is the same as the VSS and is accurate if you have the right size tyres etc.

 

Sorry. I forgot to say you can only use the in car GPS to check if you have the factory navigation.
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Use a hand held GPS to compare. Maybe a second hand held to check the first one?

Kinda like having two watches- which one is right if they disagree?

 

That's what I was thinking too!
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The ETM value was about 4.5 MPH less than the speedometer at 80 MPH.

Lots of info including DTCs, have to spend some time digesting it all.

 

Ignore the DTCs (unless you know which years they were stored?). Or get a tool that clears them and see which come back.

I think the 4-5mph is software deliberately making gauge read more.

 

Here is a good article about speedometers.

Speedometer Scandal! - - Car and Driver

I am sure my 04 STR reads high. I often set my cruise control at 80 travelling on 75mph roads and the troopers/DPS don't even look at me. I've been using the Waze app, and at 80, the GPS reading states I am travelling at 76-77mph.

I"ve also noticed that the faster I go, the greater the speed difference between my speedometer and my GPS reading. (Explained in the article...)

 

(What a strange title. Hardly a Scandal.)

 

They could make a reality show about speeders who aren't really speeding! Would that be scandalous?

 

You're not judged by your car's speedo...

Sounds like a boring show!

 

A 'conspiracy theory' would be more accurate but just as boring. The article is way out of date. This is more relevant

Speedometer - Wikipedia, the free encyclopedia

 

Maybe "International Standards agreed by National bodies help motorists avoid speeding." Wow. Scandal indeed. Conspiracy, pretty well admitted. Hold the front page.