Why is GPS speed different?

[gxr]

why dont my speedometer and my GPS read the same speed?
If I am doing 90 km/hr on the speedometer my GPS says I am only doing 80?
That I can live with! But the nagging question is what does traffic police radar work on, my speedo or my GPS?
Any insight?

[YNOT]

Your GPS is likely to be more accurate. If you drive by the speedo in the car you should be safe from police radar as you will be driving under the speed limit. Car speedo's are often not accurate.

Tony

[Sir Roofy]

Ive also noticed that my gps says im slower than the speedo
and the same in the bosses car x10ks slower

roofy

[rkinsey]

Hi GXR and Ynot,

Not wanting to step on your toes Ynot as you are the guru when it comes to the mechanical stuff. I just wanted to add to your answer above.

Car GPS devices are, in general, accurate to 1 - 5 meters so it will be more acurate more consistantly as it relies on mathematical equasions to show your current speed. Where your speedo relies on a mechanical system to show your speed on the Speedo. If any part of the drive train has been modified then you will need to have the speedo re-calibrated. Have this done by the manufacturer or one of their autrhorised service agent.

Here is some info on how a speedometer works that may help you, and us, understand what to look for. http://www.explainthatstuff.com/how-speedometer-works.html

Cheers,

Rob

[the evil twin]

why dont my speedometer and my GPS read the same speed?
If I am doing 90 km/hr on the speedometer my GPS says I am only doing 80?
That I can live with! But the nagging question is what does traffic police radar work on, my speedo or my GPS?
Any insight?

Speedo readings (not Odometer just the Speedo) are required by ADR's to overread the cars actual speed by between 0 (IE be dead accurate) to up to 10% of reading + 4 KPH. They cannot indicate the vehicle is travelling slower than actual by even 1/10th of a KPH

EG if your actual speed is 80 KPH then the Speedo is required to indicate between 80 and 92 KPH.... at 100 KPH actual indicates between 100 and 114 etc. Most manuf will set 'em at around 7 ish to allow for punters to go up one size in placarded tyres and still be OK.

GPS speed is accurate to + / - 1 KPH at least 99% of the time IE if it says you are doing 80 you will be doing between 79 and 81.

GPS is equally as accurate as Radar or Lidar in the Lab as they all work on Doppler Shift of the radiated frequency BUT the GPS signal has to travel about 17,000 K's so is subject to some atmospheric and reflection errors as opposed to the Radar/Lidar which is only several hundred metres travel but has other problems to compensate for.

[boggy2006]

I've noticed this as well. At about 50kph it's fairly accurate, but by 110kph, the GPS is only showing 100kph.

[Finly Owner]

Put it simply, Many people change the rolling circumference of their tyres from factory set up to way bigger, and never have their speedos recalibrated. Therefore speedo reads incorrect.

[the evil twin]

Put it simply, Many people change the rolling circumference of their tyres from factory set up to way bigger, and never have their speedos recalibrated. Therefore speedo reads incorrect.

Yep, but need to keep in mind that bigger rolling circumference makes the Speedo read under not higher.

What going from 31's to 33's or bigger does do that is often not taken into account is throw the Odometer out when working out Fuel Consumption and Service intervals.

[Finly Owner]

Yep, but need to keep in mind that bigger rolling circumference makes the Speedo read under not higher.

What going from 31's to 33's or bigger does do that is often not taken into account is throw the Odometer out when working out Fuel Consumption and Service intervals.

I knew when I was typing that, I had it back to front, but, it was the principle I was trying to state that time.
IF you change wheel circumferences, get speedo calibrated.

[Silver]

sometimes the speedo drive gear at the gearbox end has an incorrect ratio. A cynic might say that if the Ks click over on the odometer more quickly than they should, that is a good thing for those who offer a warranty. I have seen this with new factory fitted tyres and rims in a GQ with 16s.

In the case of my Mav, however, I suspect it is because it was set up for 750 R16s and changed in aust to 10rs

[Bigrig]

sometimes the speedo drive gear at the gearbox end has an incorrect ratio. A cynic might say that if the Ks click over on the odometer more quickly than they should, that is a good thing for those who offer a warranty. I have seen this with new factory fitted tyres and rims in a GQ with 16s.

In the case of my Mav, however, I suspect it is because it was set up for 750 R16s and changed in aust to 10rs

The legislation for Aussie sold motor vehicles changed a few years back (and for most countries I'd assume). Speedos in the vehicle used to have to be calibrated to within plus or minus a certain percentage - I think it was actually 10% which is alarming - but now all new vehicles MUST overstate your speed - that is, if you're doing 100 on the speedo with factory setup, then you're actually doing less - probably more like 95 or 96. As said, changes to tyre circumference will affect this and the larger the tyre, the more understating of speed that occurs as a result.

[the evil twin]

sometimes the speedo drive gear at the gearbox end has an incorrect ratio. A cynic might say that if the Ks click over on the odometer more quickly than they should, that is a good thing for those who offer a warranty. I have seen this with new factory fitted tyres and rims in a GQ with 16s.



Maybe years ago, but the Odo's tend to be fairly accurate as it is the Speedo that is required to overread by ADR's. If I had factory tyres on my truck the Odo would be almost spot on (underreads by 6% with existing tyres) and the Speedo would be out by 9 KPH at 100 (overeads by 3% at 100 KPH).

Underreading Odo is a bitch in my case 'cause I have to do 6% more K's to hit my minimum K's for my Lease.

Nissan have a range of drive gears available if anyone does want to calibrate their Speedo's cheaply and easily.