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.
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.
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.
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.
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.
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.
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).
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.
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.