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dennis.dunbar

GPS: Nuvi 765T







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Before I start I would like to thanks everyone for a great website.

I've noticed that the speed indicated on my rental cars are never the same as what the GPS displays. Most of the time the speed indicated on the GPS is lower than the rental car. Is this normal. which one should I be using as the true speed.

Dennis
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christo2262

GPS: TomTom Rider







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Short answer: neither Smile

Long Answer: (still neither)

Cars by design over indicate road speed for legal, practical and design reasons BUT it is measuring speed on the road or rather it counts the RPM (speed) of the tyres

GPS measure a calculated speed based on signals from space. The accuracy is determined by update interval, terrain and speed. It is not a measure of road speed.

Police radar on the other hand is considered to be an accurate measure of true road speed.

GPS speed is probably closser to the Police Radar speed than the Car speedometer; but is it higher or lower? ... only the policeman can tell you.

Enjoy the drive, cheers Chris
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dennis.dunbar

GPS: Nuvi 765T







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Thanks. The reason I ask is I got pulled over for speeding and I was using the GPS instead of my car because I figured it would be more accurate as I have larger tires on my Jeep. Well either way I guess I will just have to slow down a bit.
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jenhsuanchien

GPS: Garmin nuvi 260







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car speedometers are often inaccurate or have a safety buffer preset inside to stop people from suing manufacturers for the user's own negligence.

also, tire and wheel size affect it let alone the tire pressures
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life.by

GPS: Garmin Nuvi 3790T







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It is really good to know the information. I find my speed on GPS is always slower than car speed indicator.
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erinninn

GPS: Garmin nuvi 250w







Post Garmin's speed is very accurate     
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I found out that some Garmin's 260 speed is very accurate. I benchmarked against the speed measuring sign planced beside the road and it is pretty accurate.
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brady58

GPS: Garmin Zumo 660







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I have never driven a car that indicates accurate speed. Most manufacturers over exagerate by around 10%. dont forget that GPS speed is not instantanious
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haza121

GPS: Garmin 765T







Post Accurate     
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I thought the same thing after I operated my GPS in a Camry. The Camry was always off a few. Yet once a used the GPS in a civic which has a digital dash, the speed limit was 100% accurate. its the same deal using two different models.
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cbarsoum

GPS: Mio 701







Post gps speed     
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I agree with the first reply, the GPS is not always accurate and the car is not always accurate, to be safe always drive couple km less to avoind being fined for speeding specially around school zones with radars.
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johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3







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Have checked GPS against speedo in Honda Jazz and Toyota Prado, and only 1 KPH difference to GPS on both vehicles.
Also checked against overhead speed readout on Hume Hwy south of Albury/Wodonga, (serviced this week) and readings are same as overhead display.
Think the GPS unit will be pretty accurate as they update every second, so don't think you speed will alter much in that time frame.

Also cheacked against speedo in a holden astra and speedo was 8 kph low.
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Retread24800

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Post Speedo vs GPS     
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Just come back from 3000km trip to Germany from SW france.
The Speedo was (over straight roads) always 5% faster than the GPS
Over twisting roads the GPS tends to measure 'distance made' divided by time =speedmade good or velocity. not as the speedo does measures the speed over the road so again the GPS will give an even greater differential, ie about 10/15% lower than the speedo so on twisty roads this could result in a 'fine' as the radar read speed would be closer to the speedo speed.





jason.learmonth

GPS: Skoda Columbus (RNS510)







Post GPS Speed Reading     
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Most cars, particularly the majority of late model seem to have conservative speedometers. And not just a little, like 10-12km/h at freeway speeds of 100-110km/h. To travel 110km/h, according to my dash I sit the speedo needle at 118-119km/h and I do not slow down when passing police traffic units with radar guns or mobile speed cameras that have planted themselves on the side of the road. I feel quite confident that the GPS speed is accurate to within 1km/h when operating on an arterial road with a decent view of the sky (more satilites = more accurate).
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kxd

GPS: VW RNS510







Post Speedo Variations     
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I think you need to remember that the Australian Design Rules (ADR's) only require a 10% variation in accuracy on vehicles. Add to this the diffences in tyre size, etc., and you can easily be +/- 10-15kph off your speedo reading.

I know from experience that my speedo is -3kph at 100kph, so I am really only doing 97kph. This i've checked with various GPS devices which show a TRUE Speed Over Ground (SOG) reading. Now the caveat is that this is only when I have brand new tyres of the correct size fitted. As the tyres wear, the speedo reading get more inaccurate as the rolling diameter increases, ergo toward the end of my tyre life (around 45K Km) I am regularly -7Kph off my speedo reading. Good for the speed camera's but something to keep bearing in mind.
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johmazz2006

GPS: Garmin Nuvi 765, Garmin 3950LMT, Garmin 2455LMT, Navman40, Garmin StreetPilot 3







Post Re: Speedo Variations     
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Think you may have things the wrong way around.
Tyres wear, circumference gets smaller, wheels rotate faster at the same actual speed, so if your speedo says 97 kph true and indicated says 100 kph, when your tyres wear or get under inflated, your actual speed at indicated 100kpa would be less than the 97 kph.

Easy to cross check. If you have a tachometer as well as speedo, pump tyres to 40 psi, run at speedo 100 kph and check revs, then let tyres down to 20 psi and run speedo at 100 kph and check revs again.
Revs at second test will be higher due to tyres turning faster due to smaller circumference at lower pressue.
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