Beat speeding fines with GPS

[steeba]

My Forester is the same as well- earlier model than those mentioned though!

[shonkylogic]

Must be a Subaru thing, the speedo in my Subuaru Outback also reads 10% more than the actual speed. I just thought it was shonky.

I unwittingly tend to compensate for the 10% by driving on the highway with the needle sitting just under 120, which is really about 108kmh. I just need to remember not to do this when I'm driving a different vehicle.

[Coffee and Cache]

Well the ADR reg should ensure that the speedo reads no more than 0.1 kmh under so you don't get caught for speeding and no more than 4kmh over. 10% over would seem to be outside the current limits.

[Captain Terror]

Like tronador said, why can't they just be accurate? Theres a new idea. Takes the guesswork out of it doesn't it. I understand hire companies to an extent, and obviously older vehicles deteriorate and well, you get that. Like my 1990 Corolla reads fast I may think I'm doing 125, but it's only 110

[Geodes]

Like tronador said, why can't they just be accurate? Theres a new idea. Takes the guesswork out of it doesn't it. I understand hire companies to an extent, and obviously older vehicles deteriorate and well, you get that. Like my 1990 Corolla reads fast I may think I'm doing 125, but it's only 110

Because, basically, the speedo can only use the rate at which the wheels are going round, combined with the diameter of the tyres, to work out an estimate of the vehicle's speed. If the tyres are worn by 3mm, their circumference (and, therefore, the speed estimate) is reduced by around 1% (for a tyre of around 600-700mm diameter). If the tyres are under-inflated, their effective circumference may be reduced further - although, as C&C commented, there are a few ways of viewing this (if they were like tank/bulldozer treads, I don't think it would make any difference - but there is a fair bit of resilience to rubber tyres and, I think, the circumference is increased as they stretch to achieve full inflation). Another factor, which would account for greater speedo errors at higher speeds, is slippage; I'm not sure if it's a big contributor to the error, but at higher speeds and more slippery conditions (rain, ice, oil, dirt) a (very slight) loss of traction could contribute to a small percentage of speedo error creeping in.

I thinks it's definitely wise of the manufacturers to account for some of these possibilities and have their instruments reading high, otherwise we might all be contributing even more to the revenue raisers (who are only doing it for our own good!) - although errors greater than 5% would seem a little excessive and totally undesirable as they would reduce confidence in the unit totally.

PS As other have indicated, if you want something more accurate - use your satnav/GPSr - I trust mine implicitly and always use it to set my cruise control (my speedo reads a couple of kph high at 100) and haven't had a speeding ticket while using it yet.

[squalid]

This tolerance discussion is interesting. Three of our present and past Toyotas (Toyotae?) register within about 1 km/h at 100 km/h on both Garmin and TomTom. So that's the needle just on the higher edge of the mark on the dial.

[Bewilderbeest]

This tolerance discussion is interesting. Three of our present and past Toyotas (Toyotae?) register within about 1 km/h at 100 km/h on both Garmin and TomTom. So that's the needle just on the higher edge of the mark on the dial.

I've had the same experience, going through the speed check on the Hume (down in Vic) in the Commodore, the speedo, GPS, SatNav & speed check all within 1km of each other.

[Captain Terror]

PS As other have indicated, if you want something more accurate - use your satnav/GPSr - I trust mine implicitly and always use it to set my cruise control (my speedo reads a couple of kph high at 100) and haven't had a speeding ticket while using it yet.

As do I. Just as I set my watch by my GPSr. After all, they use the most accurate clocks known to man.

This tolerance discussion is interesting. Three of our present and past Toyotas (Toyotae?) register within about 1 km/h at 100 km/h on both Garmin and TomTom. So that's the needle just on the higher edge of the mark on the dial.

Toyotae! I like it

[Big Matt and Shell]

From ADR regs

The speed indicated shall not be less than the true speed of the vehicle. At the test
speeds specified in paragraph 5.2.5. above, there shall be the following relationship
between the speed displayed (V1 ) and the true speed (V2).
0 ≤ (V1 - V2) ≤ 0.1 V2 + 4 km/h

I've never looked that up before but that is a substantial margin for error. Lets take a car that has an indicated speed of 100 but a true speed of 91.

0 ≤ (100 - 91) ≤ 0.1(91 + 4)
0 ≤ 9 ≤ 9.5

Legally that could be a very big difference between cars Remember that next time someone goes past you on a highway and your immediate thought is "they must be speeding" perhaps it is just you travelling well below the speed limit!

[Coffee and Cache]

Yes, the range is fairly high isn't it. No doubt to cater for all those effects Geodes mentioned and still indicate at or above the actual speed. With so many cars getting integrated GPSr's these days, it should be possible to calibrate the speedo off the GPS. You would still need the speedo mechanism due to it's more responsive nature, but it should be possible to get it to within 1% or better. Did a trial recently with a GPSr with integrated dead-reckoning where the odometer (pulse) output is calibrated from the GPS, to keep position updates in tunnels etc. Something similar would provide a dead accurate speedo, and even compensate for tyre pressure and other effects. Just a matter of time I guess.