Help - not Automobile minded

[rogerco (Roger)]

So the odometer says 5.2 when I pass the 5 kilometre sign post on the highway (tested it on two sections) so this means that when my speedo says 104 kph I am actually doing 100kph right?

Can anyone confirm this or is it the other way round?

Thanks

[tilbrook@rbe.ne (Justin Tilbrook)]

Originally Posted by rogerco

So the odometer says 5.2 when I pass the 5 kilometre sign post on the highway (tested it on two sections) so this means that when my speedo says 104 kph I am actually doing 100kph right?

Can anyone confirm this or is it the other way round?

Thanks

Hi Roger,

Standard practise by car makers to make them read 4kph slow. our Hyundi SUV reads the same as yours.
I checked it against my sat-nav.

Cheers,

Justin.

[mithrandir (Andrew)]

Originally Posted by rogerco

So the odometer says 5.2 when I pass the 5 kilometre sign post on the highway (tested it on two sections) so this means that when my speedo says 104 kph I am actually doing 100kph right?

Can anyone confirm this or is it the other way round?

Roger, depending on how the speedometer and odometer work it might be correct. If the system is fully digital then it should be. If it is mechanical then all bets are off. You might find the ratios change non-linearly with your speed.
If you have a GPS and a straight road to test on you will get a better comparison.
Always remember that your true speed changes with the rolling diameter of the tyres. The more you pump them up the faster you are traveling for the same indicated speed.

[brian nordstrom (As avatar)]

Yes Justin is right , I tested this over and over and over again on my 5000 km trip from Darwin to Perth between 400km and 900km a day on average , my ute is a 2011 model and yes it was that far out every time I tried , its a loooong way and I amused myself doing this .
Brian.

[Allan_L (Allan)]

Oddly enough for me, the trip meter (because the odo only does whole digits) is always spot on precise with the 5 kilometre tests.
But like everyone else, the speedo reads 4 kilometres per hour higher than the GPS (confirmed by the occasional digital speed test readouts on some roads).

As mentioned by mithrandir, I suspect the tyres may not be standard, or properly inflated.

[rogerco (Roger)]

Yes I was watching the GPS and it was showing 5 kph slower all day during a trip to Canberra and back. Its an old Mitsubishi Nimbus, with mechanical odometer but with a tenth digit. It has had a variety of tyres and I was wondering as most people seem to be passing me.

Thanks anyway.

[Camelopardalis (Dunk)]

That also means that they're gaining km on the clock when they shouldn't be

[rat156]

Originally Posted by mithrandir

Always remember that your true speed changes with the rolling diameter of the tyres. The more you pump them up the faster you are traveling for the same indicated speed.

Whilst the basic assumption here is right, i.e. the rolling circumference of the tyre does determine the relationship between road speed and indicated speed, pumping your tyres up will not change the indicated speed noticeably unless they were dangerously under inflated anyway.

Always make sure your tyres are properly inflated, they are the only contact between you and the road. BTW, don't go by the tyre placard in your car, ask the tyre technician when you get them fitted, modern tyres, particularly the very low profile tyres now fitted as standard to many cars need much higher pressures to avoid damage, typically I run 36-38 psi, sometimes up to 40 psi if I have some high speed driving ahead.

Cheers
Stuart

[raymo]

It is not standard practise for car makers to make speedos run 4klms,
or any other amount of klms slow. In fact efforts are made to avoid
this, as motorists can get caught for unknowingly speeding. You find
similarities in inaccuracy within makes because they frequently source their speedos from only one, or at most two suppliers. Mechanical speedos of yesteryear varied wildly[ my 1976 Chrysler Centura was
12klms slow at 100kph, my 1964 Jaguar 420 was 14klms fast at 100kph. Speedos have improved steadily, and my 2001 Astra is 4klms
fast at 100, and my 2006 Corolla is 1 klm fast. Almost all currently
produced cars are made to be either spot on, or at most 2 or 3% fast at 100kph, and for the most part they succeed. The inaccuracy of mechanical speedos is a linear progression, provided that all the parts
are working properly. As there was not a mechanical connection
between the cable and the needle, stickiness in the mechanism within
the speedo itself could cause the linearity to be lost. I was involved in
complying imported cars, and one of the most important jobs was testing, and if necessary, recalibrating their speedos, and fitting the correct
tyres so the speedos were as accurate as possible.
Sorry to ramble.
raymo

[BPO]

Not much help from me, I'm afraid. I had to look up "automobile." It means "car"!

[Kunama]

The national standard at the moment requires that a speedometer fitted to a motor vehicle :
1. Must not under read, that is, at a true speed of 100kmh the speedo must read 100 or more and
2. The speedometer must not over-read by more than 10% + 4kmh of the true speed, therefore at a true 100 the speedometer must read between 100 and 114kmh.
There is no actual set minimum 'over reading' of +4 kmh

The tests are done with the 'normal' tyres fitted to that vehicle by the manufacturer. at normal pressure recommended by the manufacturer + 0.2 bar and at 23 + or - 5C

P.S. Toyota Echo owners have one of the few cars that actually reads correct speed.

[pdalek (Patrick)]

I remember back in the '70s when some taxi operators used to fit oversize rims with over inflated tyres when taxi meters were calibrated.
The alternative was swapping out the speedo drive gear, but this requires faking the lead crimp security seal on the gear carrier.

[GeoffW1 (Geoff)]

Originally Posted by rogerco

So the odometer says 5.2 when I pass the 5 kilometre sign post on the highway (tested it on two sections) so this means that when my speedo says 104 kph I am actually doing 100kph right?

Can anyone confirm this or is it the other way round?

Thanks

I'm not altogether sure your question (as posed) actually got answered

Yes, your speedo unit says you went further than you did really. So when it reads 156 kph you are only doing 150 kph, no worries

Apparently this is all to standards.

Cheers