Thread: For fast drivers who use the speed limit leeway..

The odo in my RRc "Vague" is less than 1% out the speedo is spot on at 100-110KPH, around 3-4% under reading at 80KPH and almost 10% under reading between 50-60KPH with more error the slower you go. It is a mechanical cable with centrifugal weights and spring type.

The SIII "FFReddy" speedo over reads by 10 KPH across the speed range while the odo is correct. Can't be wrong size tyres, because the ODO would be out and the speedo error should increase as the speed increases.

1) There's a problem with the +/- 10% ADR comments: the date of application. It's relevent to vehicles manufatured after 1988 BUT is superseded by ADR18/03 which applies to vehicles manufactured after 2006, and ,apparently, ALL vehicles on the road after the start of 2007. Dunno quite how they'd enforce that. Anyhoo, the newer ADR says

5.3. 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

In practical terms, your speedo must show a speed of between 114 and 100km/hr at a true speed of 100km/hr. At 110 the acceptable range would be 110 - 125.

2) The odometer will measure distance precisely, regardless of speed. The speedo uses compensatory mechanisms (either mechanical, electromechanical or electronic) to indicate the speed. This can vary quite a bit. I did a speedo swap in my old Stage 1 and went from indicating about 15% higher than actual to being almost precisely correct. But the was no change in the rate of rotation of the cable and the odometers were equally accurate.

Personally, I think the simple approach is:

a) Measure the accuracy of your speedo using a reliable GPS.
b) Assuming your speedo is consistent in its readings, take note of the indicated speed required to achieve a true speed of just under the legal limit.
c) Drive at the indicated speed which gives your maximum legal speed. - Easy!
d) If using a cruise control, note the maximum amount of surge (say 5km/hr) and set a speed low enough that even with a surge it will not result in a fine (e.g. set true speed of 98km/hr if your cruise can surge by 5km/hr).

If your speed is not reliable (like my Defender's, which seems to vary up to 5km/hr in its indicated speed when the true speed is 100km/hr) I would recommend a GPS indicating the speed.

On the other hand, our VW Transporter (electronic speedo) shows true speed +4km/hr at all speeds (104 indicated = 100, 64 indicated = 60). Cruise control will over-run 4-5km/hr. I normally set to 104 indicated and look ahead!

EDIT:

See here: http://www.trafficlaw.com.au/speedos.html for a lawyer's view on the Victorian law. Bear in mind that reading this discussion will probably invalidate your defence, as does knowing that the speedo is allowed a 10% tolerance in the older ADR!

If you are wondering why some drivers are poking along at 94k, this could be why. I took my daughters Hyundai for its first service and complained that the speedo was 7 k out. Doing 100 by speedo,the GPS and SATNav said 93k. The lady said yes my Subaru is the same. As it is within Australian Design Rules, it is ok as long as it does not read over 100 when you are doing 100, and it can be anywhere below. My D1 is only 2 k out.