Car speedo

[Henstepper]

Ive noticed using waze and google maps the speed is 5-10km slower than whats displayed on my car speedo. I downloaded a speedo app and same result. Is my car speedo out or is it the gps? 

[Tony Prodigy]

All car speedos have a speedo error built in above 40 Km/Hr.  They do it so you don't get pinged for speeding. So it's not uncommon for your speedo to read 105 when in fact you're doing 100.

Have a watch of this. It should enlighten you a little.

https://autoexpert.com.au/posts/how-accurate-is-my-cars-speedo

 

Edited August 31, 2021 by Tony Prodigy

[Hiro]

I reckon the speedo in my work D23 Navara is right on the max allowable limit - I can safely fly by roadside cameras and cops with the cruise on 115 in a 100 zone without them batting an eyelid.

Utes/4x4s etc tend to be the worst offenders too, although whether that is to accommodate the large proportion of buyers who put chunky offroad tyres on them or not is up for debate.

[Tony Prodigy]

18 hours ago, Hiro said:

I reckon the speedo in my work D23 Navara is right on the max allowable limit - I can safely fly by roadside cameras and cops with the cruise on 115 in a 100 zone without them batting an eyelid.

Utes/4x4s etc tend to be the worst offenders too, although whether that is to accommodate the large proportion of buyers who put chunky offroad tyres on them or not is up for debate.

I've been driving dual cab commercial vehicles for ages and my latest Colorado has significant speedo error. If I recall correctly as it's been a while since being on a freeway, when i'm at 110 on the speedo, it's actually around 102. I'll have to record my findings next time and report back.

For those who must put crazy huge 4WD wheels and tyres, well forget reading the speedo all together lol. Stick to GPS.

[nzzr6]

I'm pretty sure my old 2003 camry had no error built in whatsoever. What you saw on speedo was what you got

[Tony Prodigy]

3 minutes ago, nzzr6 said:

I'm pretty sure my old 2003 camry had no error built in whatsoever. What you saw on speedo was what you got

It's hard to say unless you've had it against a GPS based speed, or if you've been caught speeding too I guess in which case the copper will disclose your exact speed, in which case we'd deny it with a puzzled look on our face lol...

I'm not sure how far back this speedo error thing goes, but I would assume a 2003 car would have it. My M3 has a rather bigger than usual error I suspect, but I will have to confirm this next time I get to drive it against my google maps. It's puzzling for sure.

[Hiro]

On 9/21/2021 at 3:59 PM, nzzr6 said:

I'm pretty sure my old 2003 camry had no error built in whatsoever. What you saw on speedo was what you got

To an extent that's just luck of the draw.  There's no guarantee that _all_ 2003 Camrys have perfectly accurate speedos, but considering under the laws of the day an error of 0 was perfectly acceptable it would have happened a lot more often than you think purely on average

 

On 9/21/2021 at 4:04 PM, Tony Prodigy said:

I'm not sure how far back this speedo error thing goes, but I would assume a 2003 car would have it. My M3 has a rather bigger than usual error I suspect, but I will have to confirm this next time I get to drive it against my google maps. It's puzzling for sure.

Speedo errors have existed as long as speedos have.  ADR18/00 (the first instrumentation ADR) was introduced in 1988.  ADR18/03 (the third revision of ADR18, which was introduced in 2006) was the changeover between the old (+/-10%) margin and the new (+10% + 4km/h) margin.

[Sebastian Woodhouse]

Should note:  ADR specifies the maximum error.  The biggest change is now under any condition/test a vehicle as delivered by the manufacturer cannot have a true rod speed that is above the indicated speed.

The are also some provisions around the visual accuracy of reading the instruments as well which is where the 4km/h typically can be explained off.  From memory the 4km/h is arbitrary because it’s the largest whole number under 5, and the wording is something like “the operator must be able to determine the the reading within x% of the scale”.  Which broadly means if you see the needle half way between the 90 and 100 marks you must be doing less than 95. But if parallax error due to seating position means you are seeing the needle position on scale differently (like your passengers do) then you would still have a margin of safety.  

This is why often a digital readout speedometer is “more accurate” than the gauge style because it doesn’t actually have to deliberately over read to get around visual interpretation of needle position.

Obviously the percentage error is also about mechanical (and digital relationships between rotating items).  Some cars have incredibly accurate speedometers that are near perfect to 40km/h then from that point adopt an exact 4km/h over read.