[jigar1791@gmail]

Background -

I drive a 2018 Ciaz Petrol MT. Recently, I was on a 500 km drive from Mumbai to Ahmedabad, and had used the cruise control feature quite a times on the NH, especially on the Vadodara Ahmedabad Expressway(National Expressway-1).

Problem Statement -
I observed a huge difference in the actual speed while using the cruise control (here after, CC) and while verifying it with Google Maps (GPS, here after).
I had set the CC at 110, and just out of curiosity, tried to cross check the speed with GPS, which was showing actual speeds in between 95-100 kmph.

I also tried to measure my speed with a GSRTC’s Volvo, as I knew it’s speed being restricted at 95 kmph, and I somehow felt I took longer to overtake the Volvo (I’m at 110, Volvo at 90-95).

Conclusion -
This makes me wonder, is the CC speed set function showing reduced speed then it’s set at? Or is there something else?
Have fellow Ciaz owners/CC users made a similar observation?

@Mods - you may please move/merge this thread with a previous one with same topic, if available.

Regards,
Jigar Shah

[paragsachania]

Originally Posted by jigar1791@gmail

Background -

I drive a 2018 Ciaz Petrol MT. Recently, I was on a 500 km drive from Mumbai to Ahmedabad, and had used the cruise control feature quite a times on the NH, especially on the Vadodara Ahmedabad Expressway(National Expressway-1).

Problem Statement -
I observed a huge difference in the actual speed while using the cruise control (here after, CC) and while verifying it with Google Maps (GPS, here after).
I had set the CC at 110, and just out of curiosity, tried to cross check the speed with GPS, which was showing actual speeds in between 95-100 kmph.

I also tried to measure my speed with a GSRTC’s Volvo, as I knew it’s speed being restricted at 95 kmph, and I somehow felt I took longer to overtake the Volvo (I’m at 110, Volvo at 90-95).

Conclusion -
This makes me wonder, is the CC speed set function showing reduced speed then it’s set at? Or is there something else?
Have fellow Ciaz owners/CC users made a similar observation?

@Mods - you may please move/merge this thread with a previous one with same topic, if available.

Regards,
Jigar Shah

This has nothing to do with Cruise Control but this is exactly how the numbers are displayed/pointed in the speedometer console which is always higher than the GPS speeds which is a measurement of true speed.

This is mostly the case with almost all the cars, few with a very high variance too. Ideally, there is a difference (speedo error) of around 4-6% and that means if your speedometer shows 100 Kmph, your true (GPS) speeds will be around 95-96 Kmph.

You can try by just setting the navigation on Gmaps and then compare that with the speed shown on the car's instrument console and you will notice similar error.

When you set the CC to 100, it will maintain the car's speed at 100 but with respect to the car's speedometer reading. And hence, it will be more than what your GPS device is to indicate.

There is also reason why this is done. You can read it on this thread:

Speedometer Calibration | Govt says vehicle should overstate speed

[SamarthMehrotra]

Most cars have speedometers that generally have 5-10% error on the positive side.
The Cruise control accordingly sets the value based on the speedometer calibration.

Google maps shows you the speed based on GPS calculations. Hence, there would always be a difference unless your car speedometer is very accurately calibrated.

Some theorists say that Car manufacturers introduce this positive error deliberately to make the car feel faster.

[msdivy]

Speedometer reading is also dependent on tyre size. It is close value. But GPS reading accurate.

[paragsachania]

Originally Posted by msdivy

Speedometer reading is also dependent on tyre size. It is close value. But GPS reading accurate.

True. That is the only way you can get the reading as much closer to true speeds. In my case, I have set of tires (upgraded) due to which the speeds displayed in the console are nearly accurate to GPS speeds.

Due to this, there is however a difference in the total distance travelled (Odo or trip meter) at the end of that long journey and hence even the DTE or Average FE displayed is lower because everything is related on how the speedometer is calibrated.

[condor]

Parag's correct there. Your cruise control speed is synchronized to the vehicle's speedometer.

My Storme shows a 4 kph higher speed compared to the automatic speed displays. Have checked it quite a bit against a MapMyIndia PND.

[Shiftlock]

The speed shown on a vehicle's speedometer will always be over stated by 3-5%. The most important reason for this is to ensure safety on highways and to make sure that people adhere to the speed limits. By over reading the speed by 3-5% they make sure that people driving at speeds close to the legal limits are actually never crossing the limit. This practice is very useful in international countries where speed limits are usually heavily enforced.

Google maps will usually show a more accurate value however even that is not 100% your true speed. The reason for this is that there is a small delay between the signal traveling from your phone to Google's satellites and back. While traveling at high speeds even a small delay can result in a 1-2% error.

Hope this clarifies your doubts

[Jeroen]

As many members already mentioned; perfectly normal.

One small addition; your GPS speed will only be accurate when you are driving at a steady pace. It is very unreliable during acceleration and braking as it tends to lag behind.

Anorak fact: GPS speed is derived from using doppler phase shift calculation. Not distance over time as most internets will tell you.

Jeroen

[Bibendum90949]

Originally Posted by msdivy

Speedometer reading is also dependent on tyre size. It is close value. But GPS reading accurate.

Speedo calibration from factory itself is way off(deliberately though) the true vehicle speed. Add to that a tyre up size that has a higher variance from stock will further add to the positive error %.

On a related note, I don't think manufacturers with different tyre sizes on their different variants of the same model do the calibration accordingly either.

[haisaikat]

In my older car Getz Prime the Speedo and GPS were almost same by 1-2 KM/hr difference considering it was 2007 manufactured.

In my 2013 Hyundai i10 its a little more difference.

However in my 2019 Crysta its exactly 10% less in GPS than on the speedo but the distance calculation almost matches with GPS indicating Speed is overstated but distance is calibrated with regards to my 16 inch tyres.

Ideal solution is after the warranty expires, to get a aftermarket digital instrument cluster which reads this data correctly from OBD and shows accordingly. OBD knows the correct speed is what I have read.

[paragsachania]

Originally Posted by haisaikat

OBD knows the correct speed is what I have read.

The OBD too is likely to show you the same speed that the speedometer is going to display. No difference there unless I am wrong.

The only way to see real speed is by increasing the circumference of the tires to compensate for that percentage of speedo error. But this will affect your distance covered readings too since the calibration is based on number of rotations the wheels make.

[gkveda]

Originally Posted by paragsachania

The only way to see real speed is by increasing the circumference of the tires to compensate for that percentage of speedo error. But this will affect your distance covered readings too since the calibration is based on number of rotations the wheels make.

You will not get exact upsizing of tires to indicate accurate reading of speed. Almost impossible.

Right approach would be to take it to local meter repair shop and adjust the calibration resistor(a small component with adjustable screw on top, in the circuit) in the speedometer circuit to increase speed display by 10%. This can easily be done.

But, I would not recommend this since it does not mean anything to driver unless he is on a track to measure his or car capabilities on top speed.

[paragsachania]

Originally Posted by gkveda

You will not get exact upsizing of tires to indicate accurate reading of speed. Almost impossible.

Agreed but with the current set of tires that I have upgraded to, which are beyond the recommended size, I am get near accurate speedo reading.

Right approach would be to take it to local meter repair shop and adjust the calibration resistor(a small component with adjustable screw on top, in the circuit) in the speedometer circuit to increase speed display by 10%. This can easily be done.

But, I would not recommend this since it does not mean anything to driver unless he is on a track to measure his or car capabilities on top speed.

In the first place, my responses do not indicate an approach to meet the requirement at all. Just saying that tire circumference plays a major role to override the calibration and show you near true GPS speeds.

In my case, the recommended upsize for 185-65-R15 were 195-60-R15 and 205-60-R15 while I settled for 195-65-R15 retaining the same aspect ration albeit wider tires and hence my experience.

[Jeroen]

In theory the easiest way to get a more accurate speed reading is to get your speedometer recalibrated. Very simple, in many cases, literally as simple as turning a small potentiometer with a screw driver.

Many years ago, the speedometer in my Alfa Spider broke. I took it to a specialist company who fixed it and had it calibrated to indicate true speed. Since, my Alfa speedometer and GPS speed reading are identical.

What they do is hook up the speedometer to a calibrated input device that simulates the driving speed. On older cars, many speedometer were mechanically driven. These days they tend to be electronically, usually some sort of electrical pulse. The frequency determine the speed reading. So you need a device that can simulate either the mechanical or electronic input into your speedo meter. And it needs to be calibrated obviously.

Obviously, the calibration is valid for only a specific tire size. (Tire pressure is a factor too, but in practice will hardly show, unless the tires are badly over or under inflated).

The biggest challenge, on many modern cars, is getting the speedometer out of the dash board. On my Alfa it is a five minute job. On my Mercedes W123 I can do it even faster. But on my Jaguar it will take several hours!

Modern cars can be notoriously difficult when it comes to removing the speedometer, often the whole instrumentation pod needs to come out.

I really don’t mind at all having a difference in car speedometer and GPS Speed. I tend to set my cruisecontrol based on the GPS speed, which ensures I drive as close to the speedcamera trigger speeds as I can! We have many average speed checks here on our motorways too. Can be stretches up to 5-10 km in length.

If the speed limit is 100 km/h I will use my GPS speed reading to set the cruise control to 103 km/h. That ensure I don’t get a speeding tickets, whilst still maintaining the best speed through the speed check zone.


Jeroen

[dileepcm]

Very normal. Speedo reading is always higher in my experience. I use CC all the time. I used to set CC at 107 at 100km highway and still google maps used to read 100 in my vw golf.