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| Posted by dennis.dunbar True Speed 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 |
| Posted by christo2262 Re: True Speed Short answer: neither :) 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 |
| Posted by dennis.dunbar Re: 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. |
| Posted by GDT More... Some posts are not listed here. See all posts here: True Speed GPS Data Team |
| Posted by jason.learmonth GPS Speed Reading 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). |
| Posted by kxd Speedo Variations 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. |
| Posted by johmazz2006 Re: Speedo Variations 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. |