Speedometer Accuracy

The Speedometer on my 2016 Legacy/Liberty 3.6 is not accurate. I know that all speedos are quite often calibrated to show “‘above” the actual vehicles speed, but mine is showing “under.”

I am using a dedicated GPS operated speed HUD to compare with the vehicle’s digital speedo readout. And I will pre-empt any comments of the accuracy of the GPS by saying that I have compared my GPS with another GPS and found them to be pretty much matching. I have also occasionally passed by roadside advisory speed checkers and my GPS is a match. I am also very much aware of possible interference with GPS’s like tunnels, bridges, surrounding buildings and curves. All my comparison readings are done during a 100km regular trip on a straight highway without any of the aforementioned “interferences” …. and through pretty much rural surrounds. My tire pressures are as per the manual and my tires are still good (only 22,000 km)

What I find strange is that my speedo and my GPS are in Sync from 0 to about 90kph but around about the 90kph mark the speedo shows 4 km slower. So when my speedo shows 100kph, I am actually doing 104kph. My major concern with that is that in a 100kph speed zone, our over zealous police will ticket a driver for that.

The legislation here in Australia is very clear that car manufacturers cannot calibrate speedometers to indicate a speed slower than the actual speed of a vehicle. It can calibrate speedometers “higher” (within designated limits) but not “under.”

Here is what it says ….

From July 1 2006 a new standard began its phase in and by 1 July 2007 all new vehicles had to comply. The new standard requires that:

• The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.

What this means:

For a vehicle travelling at a true speed of 100km/h, the speedo must read between 100km/h and 114km/h. The effect of this is that many drivers will find that at 100km/h they are driving up to 14km/h below the speed limit if they rely on the vehicle’s speedo.

• The speedo must always read 'safe', meaning the vehicle must not travel faster than the speed indicated by the speedo.

I’m sure most manufacturers get a lot closer than the 14 kph as suggested above. I have spoke to Subaru Australia and they stated that ….. “ The Subaru speedometer is calibrated to ' actual speed + 5% + 2km/h ' as being the mid point in the allowable range.”

I have done a fair amount of online research on the accuracy of GPS speed readouts, and have yet to come across anything that suggests that they do not reflect a vehicles true speed but I would be interested if any other forum members would like to dispute that viewpoint. I would also like to know if anyone has an opinion as to why my speedo matches my GPS up until it gets to a higher speed.

I am assuming that my speed sensor is faulty and I will be mentioning this at my next service as a warranty item.

Take the car into the dealer and see if they can adjust it.

All of my Subaru's are exactly 2mph too fast on the speedo

2003 and 2014.

Despite what others may tell you. GPS is very accurate when you are in motion and your speed will be exactly what GPS tells you it is.

I had pretty much the identical issue with the speedo in my 2012 Legacy 2.5i LTD. (now my wife's car)

I took it to the dealer where I bought it to see if it could be adjusted...the service tech I spoke to told me that all Subaru speedometers are far more accurate that any other speed measuring device, including GPS.......... I told him that I didn't want anyone who believed that to ever touch my cars....Have never been back to that dealer since.

Since the car was getting new wheels and tires I did a little research and worked out which size corrected the issue and installed them....... Fixed the issue and had the opportunity to check the results against police radar and laser (Friends on the local PD ) Speedo is now spot on and agrees with GPS to within 1 kph

The digital speedometer, which I assume uses GPS ...

You assume wrong. The digital and "analog" speedometer displays are both driven from the same source ... electrical pulses derived from wheel/tire rotation.

Even the "analog" speedometer display is implemented using a stepper motor, which is a digital device.

You assume wrong. The digital and "analog" speedometer displays are both driven from the same source ... electrical pulses derived from wheel/tire rotation.

 

Even the "analog" speedometer display is implemented using a stepper motor, which is a digital device.

That is interesting. I wonder why the digital is correct and the analog is about 1 MPH slow? Maybe the the analog loses 1 mph in the conversion.

The Speedometer on my 2016 Legacy/Liberty 3.6 is not accurate. I know that all speedos are quite often calibrated to show “‘above” the actual vehicles speed, but mine is showing “under.”

 

What I find strange is that my speedo and my GPS are in Sync from 0 to about 90kph but around about the 90kph mark the speedo shows 4 km slower. So when my speedo shows 100kph, I am actually doing 104kph.

Hi Brisvegas,

I would say there is definitely something wrong. As you know I have an identical Australian spec 2016 3.6R Liberty & I don’t get the error you are seeing.

Just after purchasing my Liberty I checked the speedo against two Android GPS Apps; ‘DigiHUD’ & ‘Torque Pro’ (I may have also checked against ‘ActiveOBD’) & I found when the digital speedo in the Liberty was indicating 104km/h the GPS Apps both indicated 100km/h. This was on a straight flat stretch of highway with cruise control set.

At 110km/h there was still as close as I could tell the same 4km/h error in the same direction & at 50km/hr the error was less than 4km/hr but still in the same direction. So the Liberty always indicated it was travelling faster than the GPS indicated speed, as it should.

Those tests were done with the original (as new) Dunlop SP SportMaxx 050. 225/50 R18 95W tyres with 34psi in the fronts & 32.5psi in the rears.

I now have close the 25,000km on the ODO with the same tyres & pressures & notice that whenever I pass a roadworks advisory speed check I still get the same errors in the same direction but these are usually at 80km/h or 60km/h roadwork areas in NSW. I still have a fair amount of tread on the tyres & rotate them whenever I measure a difference.

Let me know if I can do any checks. Sorry I missed your PM, we have been OS on holidays & just noticed it when I logged in to reply to your post (didn't get an email notification).

Hi Brisvegas,

 

I would say there is definitely something wrong. As you know I have an identical Australian spec 2016 3.6R Liberty & I don’t get the error you are seeing.

 

Just after purchasing my Liberty I checked the speedo against two Android GPS Apps; ‘DigiHUD’ & ‘Torque Pro’ (I may have also checked against ‘ActiveOBD’) & I found when the digital speedo in the Liberty was indicating 104km/h the GPS Apps both indicated 100km/h. This was on a straight flat stretch of highway with cruise control set.

 

At 110km/h there was still as close as I could tell the same 4km/h error in the same direction & at 50km/hr the error was less than 4km/hr but still in the same direction. So the Liberty always indicated it was travelling faster than the GPS indicated speed, as it should.

 

Those tests were done with the original (as new) Dunlop SP SportMaxx 050. 225/50 R18 95W tyres with 34psi in the fronts & 32.5psi in the rears.

 

I now have close the 25,000km on the ODO with the same tyres & pressures & notice that whenever I pass a roadworks advisory speed check I still get the same errors in the same direction but these are usually at 80km/h or 60km/h roadwork areas in NSW. I still have a fair amount of tread on the tyres & rotate them whenever I measure a difference.

 

Let me know if I can do any checks. Sorry I missed your PM, we have been OS on holidays & just noticed it when I logged in to reply to your post (didn't get an email notification).

Thanks mate. Yeah, I'm convinced there is something wrong. Your car is pretty much identical to mine. I have the same tyres and near the same mileage, and like you, I also rotate them ( and I get a wheel alignment every year.) You are getting the kind of speedo readout that I expect. I'm due for a service in a couple of months and so I'll find out then if Subaru are going to do something about it. I'd be pretty surprised if they didn't, because I've been happy with the dealer's service up til now ..... and I'm not one to backdown if I feel I'm getting shafted, so one way or another I think I will get a positive result .... it will just depend on whether it gets "ugly" or not

... You are getting the kind of speedo readout that I expect. ...

I had to go for a drive this arvo so I checked our speedo against the ‘DigiHUD’ Android App to see if anything had changed. With the Liberty sitting on an indicated 100km/h in cruise control the ‘DigiHUD’ App indicated 96km/h. So I’m still getting the same results as when the Liberty was new.

I would be interested what the dealer finds.

If you compare against a GPS you'd refer to something that's measuring the actual speed.

But you need good GPS coverage and a stable mount of the antenna to ensure that the measurement is right.

A precisely measured road distance also work, and then you measure the time between the two markers on the road to calculate the speed. Earlier back in the 70's when they did helicopter monitoring of speed here they used that technique. Painted markers on the road and then measure the time with a stop watch.

I had to go for a drive this arvo so I checked our speedo against the ‘DigiHUD’ Android App to see if anything had changed. With the Liberty sitting on an indicated 100km/h in cruise control the ‘DigiHUD’ App indicated 96km/h. So I’m still getting the same results as when the Liberty was new.I would be interested what the dealer finds.

Coincidentally, today on my 100km trip along the freeway, I rechecked my speed figures and they were the exact opposite to yours. My GPS read 100kph (in cruise control) and the speedo read 96 KPH. I won't be taking it in until early July but I'll let you know the eventual outcome.

If you compare against a GPS you'd refer to something that's measuring the actual speed.

 

 

But you need good GPS coverage and a stable mount of the antenna to ensure that the measurement is right.

 

 

A precisely measured road distance also work, and then you measure the time between the two markers on the road to calculate the speed. Earlier back in the 70's when they did helicopter monitoring of speed here they used that technique. Painted markers on the road and then measure the time with a stop watch.

Many years ago they used to have "mile posts" placed at every mile interval between major cities here in Oz, and as kids it helped pass the time on road trips because with our eyes closed we played at trying to guess when we would pass the next one. The mile posts disappeared when we changed to Kms ..... I reckon a lot of them were taken as mementos and are now inside many garages