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The Pub For General Automotive Related Talk |
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18-09-2008, 07:28 PM | #1 | ||
Regular Member
Join Date: Aug 2008
Location: Melbourne, Vic
Posts: 421
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G'day all
Just wanted people's opinion on which is more accurate... the speedo or GPS sat-nav unit? I'm currently renting a car while I wait for delivery of my new one, and notice that the speedo and my portable sat-nav unit (Navman S90i) differ significantly. I didn't really notice a significant difference on my old car. eg. Speedo shows ~72km/h; GPS shows ~80km/h The difference is even greater when speeds reach 100+ km/h. I know speedos can be affected by things like tire pressure, etc., and the GPS accuracy depends on how many satellites are in view, but the GPS location seems spot on - eg. I will cross over a bridge just as the GPS unit shows I'm crossing a bridge etc. Thoughts? |
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18-09-2008, 07:34 PM | #2 | ||
Acid Rush XB Coupe
Join Date: Sep 2005
Location: a better place than you.
Posts: 2,416
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GPS triangulates your position from AT LEAST 4 satelites many many times per second. It is the most accurate method of measuring your speed, even more accurate than the "indication-only" speed detection devices our law enforcement deploys.
Your speedo is a mechanical device - it can't possibly be "accurate" and even the ADR states a tollerance of 10% on brand new cars. However, there's more than 10% difference between new and worn out tyres as the circumference reduces. Not to mention distortion of the cable (if mechanical), polar (think compass) deflections of the magnetic field driving the needle in your dash, etc. etc. Tyres are the biggest difference - but you know how TV's and computer monitors are region-specific due to the magnetic alignment and the pull of the N pole etc. deflecting and distorting the picture? Yeah well that makes a difference to spinning magnets on your speedo needle too. Doesn't affect satelite-actuated global positioning systems though.
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18-09-2008, 07:41 PM | #3 | |||||
Regular Member
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18-09-2008, 08:18 PM | #4 | ||
FF.Com.Au Hardcore
Join Date: Apr 2005
Location: Geelong, VIC
Posts: 5,267
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I'd believe your gps dude. Check your tyre sizes to see if the rent company hasnt put bigger tyres on there for some reason, check your manual perhaps for standard tyres sizes? I have a navman and a toyota echo and the digital speedo is spot on with the gps. Its also spot on with the speed indicators on the princess freeway from geelong to melb. You could drive down there one day and check them out. Good luck, this problem is not in your favour, would be nice if the gps was slower than your indicated speed.
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18-09-2008, 08:31 PM | #5 | ||
FF.Com.Au Hardcore
Join Date: Apr 2007
Location: Townsville
Posts: 1,167
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I have noticed a lot of new cars we have had indicate about 5km more than they are doing says 100 but is actually doing 95 one was doing 93 and indicating 100 manufactures wont fix it as they say its within tolerance. xr6 is showing 100 and actually doing 98
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18-09-2008, 08:33 PM | #6 | ||
VFII SS UTE
Join Date: Apr 2007
Location: Central Coast
Posts: 6,353
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i have often wondered myself.
when climbing a hill or decending a hill you are traveling vertical as well as horizontal, so that a gps for a car doesn't have an altimeter and can't measure the true ground speed, only in a horizontal distance. PS my tom tom says 100k my car say's 101/102.
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18-09-2008, 08:42 PM | #7 | ||
Youth worker
Join Date: Dec 2007
Location: Ipswich QLD
Posts: 6,892
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WOW, Ive never heard of a Speedo, indicating such a huge difference at such a low speed. At 100kmh in my Falc Im doing 96kmh GPS! Thats huge!
Agree with Baboon, Check wheels!!!
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18-09-2008, 08:51 PM | #8 | ||
Regular Member
Join Date: Jun 2008
Location: Sydney, NSW
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Nothing scientific here but from what i've been told (and it seems to make sense) the GPS will be more accurate only when you're speed is constant. E.g. You've got the cruise control set on 110 and you're cruising down the highway. When your speed is constantly varying (eg. suburban driving, 50-60km/h, accelerating and braking between lights) you're speedo will be more accurate. I believe this is both because of the slight delay and that the GPS can only locate you within around a 10m round area.
Interesting discussion though. Its intrigued me for a long time. Last edited by Lynch'd; 18-09-2008 at 08:52 PM. Reason: getting rid of the text slang |
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18-09-2008, 08:58 PM | #9 | |||
Afterburner + skids =
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18-09-2008, 09:06 PM | #10 | |||
Hmmmmmmm!!
Join Date: May 2007
Posts: 1,504
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i.e. 16" or 17" But due to a lot more tyre size's and wheel size demand in the last 10 years, it would be better for a vehicle manufacturer to err on the side of caution than to be recall every single vehicle for an "update" to correct kph readings (Showing under than the actual kph than reading spot on or over) Can you imagine if it was spot on and how many people would get booked and challange it in court, only to loose, then they try to sue the manf, due to making it too easy to speed. |
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18-09-2008, 09:08 PM | #11 | |||
Afterburner + skids =
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18-09-2008, 09:09 PM | #12 | ||
King of the Fairy's.
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I had a work Van a year or so back, and I always felt it was off because whenever I would be doing the speed limit traffic would always be overtaking. I used a GPS to get to a job on day and it was fine until 60km/h, then at 70km I was doing 66, 80 was really 73 and 90 was 80, etc.
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18-09-2008, 09:42 PM | #13 | ||
Tickford 220kw Windsor...
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My take on it is the speed reading is pretty accurate on the GPS units, but the actual refresh rate is pretty slow at every 2 or 3 seconds on my Tom Tom 910. You only need 3 points of refrence for triangulation, and as you move the refrence points track you constantly, so the speed will be right, just the refresh will be a little behind, so if your speed stays pretty constant, it will be spot on.
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18-09-2008, 09:42 PM | #14 | ||
Tickford 220kw Windsor...
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My take on it is the speed reading is pretty accurate on the GPS units, but the actual refresh rate is pretty slow at every 2 or 3 seconds on my Tom Tom 910. You only need 3 points of refrence for triangulation, and as you move the refrence points track you constantly, so the speed will be right, just the refresh will be a little behind, so if your speed stays pretty constant, it will be spot on.
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18-09-2008, 09:43 PM | #15 | ||
Tickford 220kw Windsor...
Join Date: Aug 2005
Location: Sunnyvale - Housing Commission.
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My take on it is the speed reading is pretty accurate on the GPS units, but the actual refresh rate is pretty slow at every 2 or 3 seconds on my Tom Tom 910. You only need 3 points of refrence for triangulation, and as you move the refrence points track you constantly, so the speed will be right, just the refresh will be a little behind, so if your speed stays pretty constant, it will be spot on.
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18-09-2008, 10:05 PM | #16 | |||
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18-09-2008, 10:16 PM | #17 | ||
Wheel Wally
Join Date: Aug 2008
Location: Ballarat
Posts: 883
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I personally think its GREAT that my speedo shows about 2/3k's more than whats there because if I know its a camera area I set it exactly on 100 and I know im safe.
Better than it showing 100 and your actually doing 105 etc and get booked. But yes I have to agree its PURE revenue nothing to do with safety!! |
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18-09-2008, 10:31 PM | #18 | ||
soon to be a McDonald :D
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I had a 3month old kia rio while my car was being looked at and the speedo was at 110ks and my gps was only reading 97ks
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Walking billboard Kia Rio Last edited by SBR_1; 18-09-2008 at 10:39 PM. |
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18-09-2008, 10:32 PM | #19 | ||
soon to be a McDonald :D
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double post
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Walking billboard Kia Rio Last edited by SBR_1; 18-09-2008 at 10:39 PM. |
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18-09-2008, 11:00 PM | #20 | |||
GT-P With An Ego
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Location: Canberra
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-0/+0 0-40km/h -0/+4 KM/H above 40km/h So it can't show slower than you're actually going, and no more than 4km/h over what you actually are doing. If you're doing 40 or less it's no leeway. But to answer the OP, GPS better than speedo, as long as you've got a 3D fix (4+ sats), if only a 2D fix (3-sats) the speed doesn't take into account vertical movement (e.g. going up/down a hill).
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18-09-2008, 11:10 PM | #21 | |||
FF.Com.Au Hardcore
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This is why sometimes cheaper GPSs jump after they seem to have a lock. If you have lots of sats even cheapo GPSs are quite accurate reading speed. I have checked one of mine at up to a bit over 400km/h and it was dead on. |
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18-09-2008, 11:43 PM | #22 | |||
Useless
Join Date: Mar 2007
Location: Somewhere between Melbourne Field and 17
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The one thing I tell all people when talking GPS is to remember, it is an aid to navigation, not a means, and should always be treated as such. If signal is lost, then 9 times out of 10 the receiver will continue to display incorrect information after the fact, and this information goes into it's calculations. Just so your aware.
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19-09-2008, 02:34 AM | #23 | ||
AU2 XR8
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Location: Perth W.A
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I have a TomTom and when im doing 100kph on my speedo the GPS sez im going 96-97kph by memory
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19-09-2008, 06:59 AM | #24 | ||
Rob
Join Date: Sep 2006
Location: Woodcroft S.A.
Posts: 21,777
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just to clear up another thing. as your tyres wear it will make your speedo read even faster than your road speed so even that is not a valid excuse for an innaccurate speedo.
cops know this which is why they can set fairly fine tolerances for speeding. if they do you for doing 65 in a 60 chances are your speedo was showing closer to 70 esp if your tyres were worn. |
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19-09-2008, 08:27 AM | #26 | ||
With da Warlords
Join Date: May 2006
Location: Orange NSW
Posts: 1,781
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Well my speedo is spot on correct now. According to GPS and Radar when I'm doing 100, thats spot on what my speedo tells me. Of course it helps to be able to adjust your speedo calibration lol. Originally 100 indicated was about 97-98kph true speed
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19-09-2008, 08:30 AM | #27 | |||
FF.Com.Au Hardcore
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I've been using hand held GPS systems for a while and have checked them against other units, kilometre posts (time and distance over 10km) and the Vicroads Speed Checks on Melbourne Road and Hume Hwy and they are always accurate to the kilometre. I never trust a speedo now that there's a 3km/h tollerance in my little Police State. |
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19-09-2008, 08:47 AM | #28 | ||
FF.Com.Au Hardcore
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I have found Hyundai and Subaru to over read the most.
Some ones I have tested: Hyundai Lantra 91 at indicated 100 Subaru Forester 92 at indic 100 BA Falcon XR6 96 at indic 100 08 Aurion Presara 96 at indic 100 Mini Cooper 97 at indic 100 NC Fairlane 98 at indic 100 Opel Corsa 131 at indic 143 (before I get flamed it was on an autostrada) Iveco Eurocargo MR Truck wins at 99 at an indicated 100
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19-09-2008, 09:08 AM | #29 | ||
XD Sundowner
Join Date: Aug 2007
Location: moranbah
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speedos are close with original tyres after that it is the owners responsibility to callibrate.
in saying that some cars are rediculous all my patrols have been 1-2 kms each way with original tyres towing me boat the other day in tandem with mate in new cruiser ute he kept calling me saying im flying ,was doing 110...his speedo said 125 as he tows in fouth was concerned about the economy till i gave him my gps ,not happy with 10 + km variation
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19-09-2008, 09:35 AM | #30 | |||
Afterburner + skids =
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Speed Kills. So buy an AU XR8 and live forever. Oo\===/oO |
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