Thread: Car Speedo Vs GPS

Originally Posted by Ian Smith

In my experience, car speedometers always read 5-10 per cent high, stock standard, of course.

I've checked that against several GPS' over time, and always found it to be the case.

You can further check it by timing yourself though those specially marked out sections 5 km on some highways (here in Qld, at least), but you'll find they will agree with your GPS rather than the speedometer.

Ian

Nope.

I got done years ago in my Torana. One of 3 occasions when I KNOW I was not speeding. I took it to the racq and they tested the speedo. It was within 1 mph at all the relevant speeds. The EL falcon speedo is bang on according to the gps, I've tested it on many occasions on dead straight roads at constant speed to be sure. The subaru is dead on 10% low at all speeds.

Don't even get me started on motorcycles

Originally Posted by damian

 Don't even get me started on motorcycles

My old R60 BMW had a very strange speedo. It would not even get off zero until it was doing ~30 mph and then it went straight to 30 mph, then up until about 50 mph it read +/- 5 mph but above that it started swinging back and forth like a yoyo. I replaced the cable but no change, it must have been something inside the meter unit itself. Fortunately that MC was a slow old thing and although I got pulled over a few times I never got fines or points while riding it. The closest I came was in my first out of teachers college, teaching high school in a small country town, when a new young copper, who looked like he was just out of school himself, pulled me over and said I was speeding - I knew I was doing over 50 because the speedo needle was swinging. When he found out I taught maths at the local high school he asked me if I could help him with some year 9 maths homework he was doing at the local tech. Of course I said yes and he then gave me a warning for doing 55 mph in a 45 zone. It was only later I realized this potentially amounted to some form of corruption but he never took me up on the help.

From my house in the village to the house in the city where I stay was 221km.
Every time. So, after 17 years, I replaced it with a less-old vehicle.
Now, the very same route at the very same speed, +/- one vehicle length, is 230km. Every time.

Originally Posted by Robson Valley

.Now, the very same route at the very same speed, +/- one vehicle length, is 230km. Every time.

Continental drift

Originally Posted by Ian Smith

Continental drift

If he lived in New Zealand that might be right...

Just to throw a different spin on this - I changed the gearing on my GSX-R and had it dyno tuned and also asked them to make a note of what speed the dyno was reading when the bike showed 100km/h. From that I bought a little gizmo to allow speedo calibration/changes so I did the math and set it up so 100 on the bike should actually be 100.

I go cruising by cars on the highway all the time, I'm sure they're cursing me as another speeding motorcyclist! Haha. But I hold the same speed around police and through cameras, radar traps etc. and have not got a ticket yet.

So the speed on a dyno is accurate, or close enough that you won't get a ticket in QLD if you calibrate your speedo to it!

ADR's mandate that a speedo must read fast by a certain percentage.

What is any possible reason for having deliberately inaccurate speedos??
Anyway, thems the rules. Didn't know that before.

The driver cannot claim that because the car was clocked speeding the speedo was in fact slow and indicating compliance with the law. Another way of saying it is if you are doing the indicated speed limit you cannot break the law.

GPS ha! I work for a Large transport company which has GPS locators in the vehicles and guess what? The GPS program installed that I use to see where drivers are to allocate jobs cannot get the speed zone right. Often says a speed zone is 100km/h and its in the middle of the city in streets that are 50km/h zones. Trucks can be parked in the yard and the system will say they are doing 30km/h.

I'd like to hear of anyone getting off a speeding ticket due to "but officer the GPS said....."

My daughter works for Ezi Track who supply onboard GPS tracking back to base or to your phone. A few years ago a motorist who had one of their trackers fitted was pulled over and booked for exceeding the speed limit. 18 months later (I might be wrong here on the time frame) the driver finally had the infringement overturned after a lengthy court battle after the police were proven wrong by the tracker logs. The GPS trackers in our trucks have a button on them top mark a point in the log if you want to question speed etc at the marked point.

I don't think the ADR's have changed for speedos: +-10%. Road Safety WA: 'Drop 5, Stay Alive'. Daughter: 'Add 10, Live on the Edge'. Where did I go wrong?

From ADR 18/03 http://rvcs-prodweb.dot.gov.au/files/ADR%201803.pdf


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

I stand corected.
But this will be for cars after the ADR was declared. Older vehicles (like mine) will still be on the + -10% .