Speedo accuracy

[JohnHeagney]

Hi Forum members,

I am running on 17” alloys and there is a 5% difference between the Speedo reading and reality.

So, when the speedo reads 62mph, the actual speed is 59mph. I have checked this with Waze and I also have a Speedo app on my phone. Waze is visible on this photo.

Other vehicles that I have had such as an SL Mercedes was only out by about 1mph at 60mph.

In these days, is 5% considered to be acceptable?

Thanks in anticipation!

 

[Krabby]

I imagine that the difference is not because of faulty equipment but rather purposefully done to err on the side of caution. Total speculation, of course, but I think that makes sense.

 

[Eric]

Hi Forum members,

I am running on 17” alloys and there is a 5% difference between the Speedo reading and reality.

So, when the speedo reads 62mph, the actual speed is 59mph. I have checked this with Waze and I also have a Speedo app on my phone. Waze is visible on this photo.

Other vehicles that I have had such as an SL Mercedes was only out by about 1mph at 60mph.

In these days, is 5% considered to be acceptable?

Thanks in anticipation!

View attachment 7815361

The answer is yes and no. With modern technology there is no reason a speedo shouldn't be accurate. But I believe that the law still permits manufacturers to be within 10%.

 

[CountyV8]

How did you get Waze to fill the screen like that

 

[bemax]

How did you get Waze to fill the screen like that

It’s the normal view of waze in Apple car play if you select the view for a single app.

 

[bemax]

Hi Forum members,

I am running on 17” alloys and there is a 5% difference between the Speedo reading and reality.

So, when the speedo reads 62mph, the actual speed is 59mph. I have checked this with Waze and I also have a Speedo app on my phone. Waze is visible on this photo.

Other vehicles that I have had such as an SL Mercedes was only out by about 1mph at 60mph.

In these days, is 5% considered to be acceptable?

Thanks in anticipation!

View attachment 7815361

It will be within the limits. My one, 17“ as well, showed 166 km/h at a gps speed of 160/161 km/h so this would be between 3 an 4% deviation.

 

[CountyV8]

How did you get Waze to fill the screen like that

Sorted

 

[JohnHeagney]

The answer is yes and no. With modern technology there is no reason a speedo shouldn't be accurate. But I believe that the law still permits manufacturers to be within 10%.

That’s my thought process, it could be corrected in the software.

 

[DaveB]

Hi Forum members,

I am running on 17” alloys and there is a 5% difference between the Speedo reading and reality.

So, when the speedo reads 62mph, the actual speed is 59mph. I have checked this with Waze and I also have a Speedo app on my phone. Waze is visible on this photo.

Other vehicles that I have had such as an SL Mercedes was only out by about 1mph at 60mph.

In these days, is 5% considered to be acceptable?

Thanks in anticipation!

View attachment 7815361

Most manufacturers read 3-5% higher than actual.
Ford Australia used to be 6%
Particularly annoying for those road hogs sitting in the fast lane, refusing to move over, doing an indicated 100 kmh when they were only really doing 94kmh.

 

[Tom D]

This is good news as it should read accurate with my 33“ tyres.

 

[PBD]

Hi Forum members,

I am running on 17” alloys and there is a 5% difference between the Speedo reading and reality.

So, when the speedo reads 62mph, the actual speed is 59mph. I have checked this with Waze and I also have a Speedo app on my phone. Waze is visible on this photo.

Other vehicles that I have had such as an SL Mercedes was only out by about 1mph at 60mph.

In these days, is 5% considered to be acceptable?

Thanks in anticipation!

View attachment 7815361

 

[PBD]

Mines 3 to 4 mph over to John. There is an earlier thread on this.

 

[JohnHeagney]

Most manufacturers read 3-5% higher than actual.
Ford Australia used to be 6%
Particularly annoying for those road hogs sitting in the fast lane, refusing to move over, doing an indicated 100 kmh when they were only really doing 94kmh.

Good point!

 

[Boston Boy]

what happens to the accuracy when you put on larger tires, does it somehow self adjust?

 

[DenisM]

what happens to the accuracy when you put on larger tires, does it somehow self adjust?

No !

 

[DaveB]

what happens to the accuracy when you put on larger tires, does it somehow self adjust?

The speedo will be set with the diameter of the standard factory tyres.
If you change tyre diameter then the accuracy will change.
If diameter goes up by 1" it wont have much impact, but if you go from 32" to 35" it certainly will.

Where I live the rules are fairly simple.
If the vehicle comes from the factory with the road tyres fitted
The overall diameter of any tyre fitted to a passenger car or passenger car derivative must not be more than 15mm larger or 26mm smaller than that of any tyre designated by the vehicle manufacturer for that model.
If it comes with the K02 light truck tyres
Must not be more than 50mm larger or 26mm smaller than that of any tyre designated by the vehicle manufacturer for that vehicle.

 

[Solmanic]

LOTS of variables that can also affect GPS speed readings. Trees, cloud cover, adjacent buildings, China. GPS speeds are in no way 100% accurate hence he reason they are not a legally accepted means of calculating speed by road transport authorities. Having said that, in perfect conditions the car speedo will always over-read the true GPS speed. Australian design rules (and presumably most other jurisdictions) allow for a certain % tolerance in over-reading, but 0% tolerance in under-reading by the cars speedometer.

 

[AnD3rew]

This difference is normal is most new cars. My Pajero is 7kph under at indicated 110kph my Skoda is about 5, my Jag also about 5

 

[AnD3rew]

This difference is normal is most new cars. My Pajero is 7kph under at indicated 110kph my Skoda is about 5, my Jag also about 5

From memory design rules allow a significant margin under, but basically zero margin over so manufacturers deliberately err under.

 

[bigleonski]

My last 4 Toyota’s have been at 96km/hr actual when the speedo read 100km/hr.

Changing from 32” standard tyres to 33” corrects that to almost spot on.