IS300h Speedo Accuracy.

[malcolmw]

I have been idly playing with my mobile phone and a GPS speed app.

I have found that the speedo on my other half's IS300h reads about 7% fast (i.e. at an indicated 70 you are actually doing 65mph).

Has anyone else checked this out? I think it is pretty typical.

[NemesisUK]

Very typical.

And reassuring, in that one can't inadvertently speed?

[wharfhouse]

That is normal - all cars under read a little so you can't inadvertently speed - not sure what the max number/percentage is but under reading is usually in the 5% to 10% range in my experience when checked with GPS.

[reeac]

My IS300h speedo reads 10 percent too high as judged against those roadside speed indicators. As the car is still under warranty the dealers say that they will correct this when it goes in for service and first MOT next month. My 13 year old workhorse Jazz  and my late 20 year old Jag XJ8 had speedometers with no detectable error when compared with satnav and roadside meters so I'm used to accuracy in these matters.

[Britprius]

The speedo in most cars over reads the actual speed, and this is allowed for in law. A speedo may reed over by a given maximum formula, but may not under read. This is to stop people saying they were sticking to the speed limit according to there speedo, and therefore were not deliberately speeding.
I doubt very much anything will be done to correct your cars reading as this is done electronically from the ABS wheel speed sensors, and is not adjustable.

John.

[talaipwros]

lexus speedos are about 10% optimistic ( which explains some youtube videos with some really fast IS250's )

From personal experience, for better or worse  BMW ones are much more accurate

[reeac]

We were certainly able to correct the wind speed and speed through the water on our boat instruments about 10 years ago via software and user instructions. That is why I raised the matter with the Lexus dealers and they saw no problem .... we shall see.

[Herbie]

32 minutes ago, reeac said:

My 13 year old workhorse Jazz  and my late 20 year old Jag XJ8 had speedometers with no detectable error when compared with satnav and roadside meters

I doubt very much that the speedo in any car would have no detectable error against a satnav - the below is taken from https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer

Quote

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

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.

 

[gdh300]

6 minutes ago, talaipwros said:

From personal experience, for better or worse  BMW ones are much more accurate

Agreed, and that is probably why BMW drivers end-up with reputation for tailgating; they can't understand why all these bl**dy Lexus are crawling about below the speed limit! 😉

[reeac]

9 hours ago, Herbie said:

I doubt very much that the speedo in any car would have no detectable error against a satnav - the below is taken from https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer

 

By no detectable error I mean no more than about 1 mph which is the closest that I could /can read those speedometers whilst the roadside indicators and satnav are digital readouts and so also to 1 mph. I've owned the Jazz for all its 13 years and had the Jag for 16 years so I've had plenty of time to observe these effects. Your quotation simply shows that the spec for speedo accuracy is remarkably loose ...it doesn't prove that my claims are wrong.

[Ala Larj]

12 hours ago, gdh300 said:

Agreed, and that is probably why BMW drivers end-up with reputation for tailgating; they can't understand why all these bl**dy Lexus are crawling about below the speed limit! 😉

That's being remarkably generous to BMW drovers!

[Herbie]

5 hours ago, reeac said:

I've owned the Jazz for all its 13 years and had the Jag for 16 years so I've had plenty of time to observe these effects. Your quotation simply shows that the spec for speedo accuracy is remarkably loose ...it doesn't prove that my claims are wrong.

Of course not, but it does show that your observations are very much the exception rather than the rule.

You seem to think that something is wrong with your car, hence your request to the dealers to have it sorted. All I'm saying is that there's nothing wrong with the inaccuracies of your speedo, it's designed like that for a purpose.

In fact, if your cars really are that accurate, then it could mean that as your tyres wear down and the rolling diameter is reduced, you could be breaking the speed limit even though your speedo won't show it.

[reeac]

22 minutes ago, Herbie said:

Of course not, but it does show that your observations are very much the exception rather than the rule.

You seem to think that something is wrong with your car, hence your request to the dealers to have it sorted. All I'm saying is that there's nothing wrong with the inaccuracies of your speedo, it's designed like that for a purpose.

In fact, if your cars really are that accurate, then it could mean that as your tyres wear down and the rolling diameter is reduced, you could be breaking the speed limit even though your speedo won't show it.

Do the sums. Typical wheel plus tyre is just under 600 mm. OD. Tyre wear range is 6 mm. , two sides so 12 mm variation in  OD. That's 2 percent variation in diameter and therefore in recorded speed between the extremes of wear. That's 0.6 mph at 30, 1.4 mph at the National Speed Limit on motorways. My roadside meter readings are generally In 30 zones as that's where they are most frequently located. 

[Thackeray]

1 hour ago, Herbie said:

In fact, if your cars really are that accurate, then it could mean that as your tyres wear down and the rolling diameter is reduced, you could be breaking the speed limit even though your speedo won't show it.

I might be wrong but I suspect the speedo will become more inaccurate on the high side as the tyres wear, overstating rather than understating the speed. 

For example, if the speedo says you will cover 70 miles in an hour, that figure is calculated from a fixed number of revolutions of the road wheels. That number won't change as the tyres wear. But as the tyre gets smaller the distance actually covered per revolution of the wheel will get shorter. So although the speedo says 70, the smaller worn tyres actually cover two percent less distance (using reeac's calculation). And when the speedo says 70, you actually only travel around 68.5 miles in the hour. Or have I got this the wrong way round? 

Some cars allow you to reset the tyre wear measurement to reduce this inaccuracy.

[reeac]

3 hours ago, Thackeray said:

I might be wrong but I suspect the speedo will become more inaccurate on the high side as the tyres wear, overstating rather than understating the speed. 

For example, if the speedo says you will cover 70 miles in an hour, that figure is calculated from a fixed number of revolutions of the road wheels. That number won't change as the tyres wear. But as the tyre gets smaller the distance actually covered per revolution of the wheel will get shorter. So although the speedo says 70, the smaller worn tyres actually cover two percent less distance (using reeac's calculation). And when the speedo says 70, you actually only travel around 68.5 miles in the hour. Or have I got this the wrong way round? 

Some cars allow you to reset the tyre wear measurement to reduce this inaccuracy.

The car will still travel the 70 miles in the hour regardless of the wheel diameter. The wheel diameter being less than standard will entail the wheel performing more revolutions per minute (hence an increase in displayed speed) and in total (thus giving an increased odometer reading). I was trying , however, to make the point that these changes are small enough to be ignored.

There is a more general point here which is that by identifying the magnitude of effects you can see which ones can be ignored thus simplifying life. I once attended a series of local meetings on the issue of Sustainability at which many of the prominent  contributors got themselves really worked up about trivial matters. I remember one man agonising over the fact that the Imperial ton was different from the Metric tonne .....I pointed out that the difference was only 1.6 percent which really didn't matter unless you were buying or selling coal.

 

[Herbie]

1 hour ago, reeac said:

I was trying , however, to make the point that these changes are small enough to be ignored.

Funnily enough, that's what I was trying to do. The salient points are:

• Vehicle speedos reading high is not a new thing. I'm amazed that the OP was surprised by this as it's been happening for years, but more importantly, it's by design
• There's nothing wrong with your Lexus speedo. The fact that your Jazz and your Jag appear to be so accurate is really neither here nor there, just unusual. Manufacturers design speedos to read high because it's illegal for them to read low, so the manufacturers err on the side of caution.
• Life's too short to do sums, I'm off to the pub and I'll have a pint for all the folks in this thread  

[Ala Larj]

My understanding is that most speedometers run off the drive shaft not the wheels. Surely tyre wear is irrelevant. However, I agree with Herbie. Beer is more important!

[OldTrout]

But if it is sold by the pint that pint has to measured at some stage so sums are important🙂

[Ala Larj]

17 minutes ago, OldTrout said:

But if it is sold by the pint that pint has to measured at some stage so sums are important🙂

Ever been to a weatherspoons?

[OldTrout]

Not knowingly. Is this to my credit?

As a little lad I saw pint glasses being checked at the Weights and Measures office in Stafford before getting the pint stamp.

Back on topic there is a button to press on my 450 when I have changed tyres. Something to do with satmap and position but what? 

John

 

[Ala Larj]

Yes to the first question!

Don't get me going on the difference between a pint up north and down south which seems to vary depending upon head.

I also have a button which I've no idea of the purpose of!

[reeac]

When you pay for it don't bother to count out the money, just give them a handful of change ...life's too short.

[Hadrian]

On 8/28/2018 at 10:17 PM, gdh300 said:

Agreed, and that is probably why BMW drivers end-up with reputation for tailgating; they can't understand why all these bl**dy Lexus are crawling about below the speed limit! 😉

I'm glad you said that and not me. I was told that was a fallacy. 😄😁🤣 😁

It is a fact though, that every speedo on every car indicates that you are going faster than you actually are. Assuming the car hasn't been modified in any way.

This is done deliberately, and complies with regulations......This video explains how it works in Australia and the US. It's similar here.

 

[Rabbers]

At speedometer readings of 50, 80, 100 and 120kmh respectively, the actual speeds of my RC shown by trustworthy Swiss roadside warning cameras,  are

47, 74, 94 and 112kmh.  Since I used to see the same or very similar differences for the IS300h, 250 and 200, I have never doubted that the margin of error

in calibration is intentional - and I was originally somewhat irritated by what I considered an excess of nannyism on Lexus' part.  Not that it mattered since

I quickly acquired the habit of adding the differences back to the speedometer readings and then making a further upwards adjustment in accordance with

what I know to be the official allowance on the posted speed limits.  Although these allowances vary from country, what this means is that, if you so choose,

you can, at the very minimum, add 10% to Lexus speedometer readings of legal limits without risking a speeding ticket.

[Hadrian]

If there are cameras about, I go by the speed indicated on on my Garmin sat nav.  Which isn't much help when for instance when going through road works

where there are 50mph average speed cameras, and  a long line of cars doing 45mph.

Then again, by and large I keep to the speed limits. It lets me concentrate on my driving, rather than constantly having to look for cameras, or unmarked police cars.

None of my journeys are so important that it matters what time I arrive at my destination.