Speedometer VS GPS

[Ungtiong]

yesterday a bit bored, setup 2 phones for GPS speedometer (Iphone 4 and Galaxy S2) and realize that my speedometer reading vs GPS is out..really out. Already got a hint when playing around with the acceleration (to measure 0-100) program on S2, i drove to 105km/hr before slowing down, but program never register the the 100km/hr mark.

 

speedometer cruising at 90km/hr, both GPS shows 81-82km/hr (GPS speed remains quite steady for few minutes)

speedometer 40km/hr, both GPS shows 34-35km/hr

 

search internet, found out that manufacturer always set higher (by law for saftey reasons) and up to 5% (around there...forgot the exact figure) is allowed. That is with new tryes. With worn tryes, the error can be more.

 

driving Altis 1.6 G10, My rim/tyres stock (195/65/15) ~14,000 km. NO mods (yet ). driving with 1 adult passenger and 1 baby. driver side front right very heavy...

 

Now i know, it is bugging me. no wonder i always kena tailgate, cause when i though i am 'overtaking' 100km/hr at 1st lane, actually i am just a bit over 90 (too slow at 1st lane). Also when i though the car is smooth at 110km/hr (compared to G9 altis)..its actually only slightly over 100km/hr.

 

No wonder so many ppl do 0-100km/hr "test" on the road based on the speedometer will get good acceleration figures...

 

So how to 'correct' this problem? I am not looking for exact (tyre wear will change speed anyway) but current variation is way too much. Only thing i can think off is changing rims to 16" or change tyre to 205/60/15....but sacrifice my current tyre and (spare tyre also need to change)....or can simply ignore.

 

Can tell wife its for safety reason i need to upgrade rims and performance tyres

Edited June 5, 2012 by Ungtiong

[Lattesurf]

Easiest & cheapest way is to change to slighty larger diameter tyres. There are many tyre size calculators on the net that can calculate the error difference when u change from stock to a new size. Play around with the sizes until u get 1 that "covers" that tolerance.

 

My speedo used to have about 10km/h error when on stock 16", after changed to 18" the error went down to about 5km/h.

[Detach8]

That sucks, my previous jap car (Latio) did the same thing. The speedo reading is easily 5kph off which in turns clock higher mileage and hence better FC reading. Check your tyre sizes.

 

The real-world FC of my Latio was hovering around 10-11km/l. This 5% difference could bump the FC up 1km/l.

 

My current jap (Mazda) and conti ride does not do the same thing. It reports the speed as it is correctly, with at most 1-2kph difference.

Edited June 5, 2012 by Detach8

[inlinesix]

  On 6/5/2012 at 2:28 AM, Ungtiong said:

yesterday a bit bored, setup 2 phones for GPS speedometer (Iphone 4 and Galaxy S2) and realize that my speedometer reading vs GPS is out..really out. Already got a hint when playing around with the acceleration (to measure 0-100) program on S2, i drove to 105km/hr before slowing down, but program never register the the 100km/hr mark.

 

speedometer cruising at 90km/hr, both GPS shows 81-82km/hr (GPS speed remains quite steady for few minutes)

speedometer 40km/hr, both GPS shows 34-35km/hr

 

search internet, found out that manufacturer always set higher (by law for saftey reasons) and up to 5% (around there...forgot the exact figure) is allowed. That is with new tryes. With worn tryes, the error can be more.

 

driving Altis 1.6 G10, My rim/tyres stock (195/65/15) ~14,000 km. NO mods (yet ). driving with 1 adult passenger and 1 baby. driver side front right very heavy...

 

Now i know, it is bugging me. no wonder i always kena tailgate, cause when i though i am 'overtaking' 100km/hr at 1st lane, actually i am just a bit over 90 (too slow at 1st lane). Also when i though the car is smooth at 110km/hr (compared to G9 altis)..its actually only slightly over 100km/hr.

 

No wonder so many ppl do 0-100km/hr "test" on the road based on the speedometer will get good acceleration figures...

 

So how to 'correct' this problem? I am not looking for exact (tyre wear will change speed anyway) but current variation is way too much. Only thing i can think off is changing rims to 16" or change tyre to 205/60/15....but sacrifice my current tyre and (spare tyre also need to change)....or can simply ignore.

 

Can tell wife its for safety reason i need to upgrade rims and performance tyres

There is a lag time in GPS. So, GPS may not be the most accurate device for calibration. In addition, human leg may also not be that consistent.

 

My best calibration device is speeding ticket from MY TP. I drove with cruise control set at 150km/h. MY TP stops me for speeding at 150km/h.

[Ghostrider]

On mine...the speed on GPS vs Speedo is fairly the same...

 

I do notice that you need to hold your speed for at least 3 sec for the GPS to get a lock on your speed...if you are accelerating the speed is not accurate.

[Skyiet27]

It should be wrong tyre size

 

 

Gps should be giving the rite reading

[Skyiet27]

But u can jus ignore

 

Anyway u r safer whereby u hv additional 10 kmh

 

Before u r caught speeding

 

Like my previous ride which is out by 10

 

Drive 110 on sg highway is ok lor

 

Either change wheel or cheaper way change mindset

 

[SimonTan]

It can be annoying to drive behind a car that has its speedo meter shown higher speed, when actually its lower, just like TS.

Therefore we can conclude that most cars on the road are actually driving ~10km/hr lower than they think they are.

 

maybe TS can change to 195/70R15 tyres.

 

[TL173]

You can install aftermarket Speedometer gauge to have the actual speed display to you.

[Deckbuild]

Last week, my dad took my car so I took his bike, on KJE, i'm going around 80KPH on speedo and ALL trucks over took me! Then I turn on my GPS watch, it's showing 70kph......

 

The OD of the wheel makes a diff, on my car, I change from 195/55/15 to 205/45/17, my GPS speed is now 100:101, before is around 100:95

[Happily1986]

GPS speed reading accuracy is dependent on how many satellites acquired in the GPS lock. Also, the threshold for GPS speed reading has to be considered. As you probably know, GPS precisin can be measured by means of radius. The best civilian GPS can go is 4~5m thereabout. Depending on your app and/or GPS device, it could be instructed to churn out a speed rating at very rough GPS accuracy of 50m radius. GPS radius or accuracy can affect speed assessments.

[Skyiet27]

Last x when I was riding krr

 

My bike was 20 slower than car

 

So my fren n I went to adjust the meter

 

If not imagine on 100 meter

 

We were act 70 cos bike was 20 slower n car was off 10

 

Lol

[Deckbuild]

  On 6/5/2012 at 7:34 AM, Chase1 said:

Thanks for your post.

 

 

-bump-

 

Nowadays alot of noob join and get banned before they reached 50 post....