Is my speedometer inaccurate?

[Psy]

I've been using my TomTom GPS alot lately, but one trend i have noticed on the TomTom display is that it shows 4 kilometers slower than my cars speedometer. Now i've been using this same GPS on my brothers car and the TomTom and his cars speedometer are on tune.

I used to have a older TomTom ONE XL with my previous car and it showed accurate speed, but when i sold that to get the Euro.. it was showing inaccuracies. I replaced the TomTom but to no avail, the problem was there.

How do i know what is going on?

Is it my GPS or my cars speedometer?

[panda[cRx]]

your speedo only needs to be within 10% accuracy under current Australian ADR rules (i have heard this tolerance may be increased soonish)

you will find most hondas (and other manufacturers for that matter) have their speedo reading slightly higher (about 4%) than the speed actually being travelled. this is all within 'manufacturer spec' and adr's so there is nothing 'wrong' with your car.

the manufacturers prolly do it to be on the safe side and avoid speeding fine complaints

[94dc2tegz]

it's probably ur speedo.

mine reads about 2ish off in my car too but it's spot on in my mate's car.

ur speedo sometimes is a lil faster by factory. or u have rims and it's a lil off cuz of that too. all in all i wouldn't worry too much about it.

[aaronng]

Accord Euros tend to read 100km/h on the speedo when they travel at 95-96km/h in actual speed.

[Psy]

hmm, okay then.

I feel kinda better that it's normal, i'm glad that it doesn't show slower which would result in a ton of speeding fines hehe.

I wonder why some carmakers show accurate speedo where some don't... should be a trend.

[aaronng]

Well, it helps against speeding fines. But actually each speedo has some inaccuracy when converting from an electrical signal to the needle movement. To get it dead accurate, would cost more and in the end makes less sense to spend that much since the speedo error changes as your tyres wear down!

[EuroDude]

Accord Euros tend to read 100km/h on the speedo when they travel at 95-96km/h in actual speed.

:thumbsup: Its typically 96km/h on the Euro's

[tRipitaka]

yep.. it's totally normal..

it'll change if u decide to get a larger or smaller wheel diamater when u get new rims too..

[EuroAccord13]

After testing it on a dyno so I can calibrate my RSM-GP to match, my actual speed is 97Km/h when the needle is showing 100km/h and the variance gets wider as the higher speed you go.
the problem lies with converting from an electrical signal to a mechanical one.
*Edit*
Okay Aaron has explained the electrical to mechanical part...

[Delta_One]

My speedo is about 4km out, pretty annoying seeing that most cars overtake me. But hey, mean's I wont be getting any speeding fines ;)

[Psy]

Ok thanks guys

[aaronng]

After testing it on a dyno so I can calibrate my RSM-GP to match, my actual speed is 97Km/h when the needle is showing 100km/h and the variance gets wider as the higher speed you go.
the problem lies with converting from an electrical signal to a mechanical one.
*Edit*
Okay Aaron has explained the electrical to mechanical part...

You have 225/50 R16's too, so you read 97. Mine on worn 205/55 R16's read 95. New 205/55 R16 read either 95-96 on those speed check cameras.

[EuroAccord13]

You have 225/50 R16's too, so you read 97. Mine on worn 205/55 R16's read 95. New 205/55 R16 read either 95-96 on those speed check cameras.


My original rims and tyre size (205/55R16) were also reading 97km/h...
Just so you know, the difference in diameter and circumference between the two sizes are only about 0.5mm for the diameter and around 2mm for the circumference.....
Again, mine was calibrated on a dyno. When I was passing by the speed check cameras (I did a few passes to confirm), it was also showing 97km/h..

[SPQR]

New 205/55 R16 read either 95-96 on those speed check cameras.

I agree that the Euro speedometer generally indicates about 100km/h when the true speed is closer to 96km/h and yes this changes as the tyre wear away. But I wouldn't necessarily trust the speed check radars as a matter of principle because the speed that shows on these might not be yours. Radar detects the strongest return signal. If you have a truck behind you but not necessarily right behind, then it is likely to reflect the truck's speed. If you use this check then make sure you're alone on the road in both directions! Also, if someone is speeding behind you, their speed is more likely to show up on the radar display. This is inherent in radar devices. The beam is not as narrow as the police would have the public believe.

When I was living in another state, I saw the police operators manual for the older radar (Kustom Signals KR-10 & KR-11 http://www.kustomsignals.com) that were once widely used in Australia. The manual specifically stated that it must not be used in built-up areas or areas with a lot of traffic. The police used it anyway.

The person who had the manual once faced a highly improbable speeding charge. He advised the prosecutors of his intention to use the information in the manual and suddenly all charges were dropped because they did not want the information to be revealed in court where it could be reported to the public.

More info at http://home.iprimus.com.au/johnhpowlton/radar/radarRipOff.html)

Note that I am not talking here about laser speed detection (which has its own problems).

[Blitzen]

get your car a data recorder blackbox, and shove the speeding ticket up the judges........... table to support innocent against speeding fines

anyway, my mates TomTom show i'm 5km/h down as well, so I push my luck to do 5km.h over everytime...

[Euro76]

No wonder many drivers overtake me and I thought every driver is a race car driver. If my speedo reads 100km/h then I see drivers passed me and I also thought they broke the speed limit. But anyway it's good for me that I won't be caught with speed cameras.

[pxrnm]

went to Sydney road on saturday at 110 Km/h but the digital speed check point only showing at 104 km/h.

This discrepancy is pretty much a pain if you are in convoy :(

[aaronng]

Rather be 4-5km/h under the speed limit than at the speed limit and have only 3km/h buffer (which disappears quickly when you on a slight downhill slope).

[tony1234]

went to Sydney road on saturday at 110 Km/h but the digital speed check point only showing at 104 km/h.

This discrepancy is pretty much a pain if you are in convoy :(
As said before it's within the tolerance allowed.not much you can do.

[martizzle]

if on the freeway, i dont look at the car speedo, i rather look at the gps speed, and hit cruise control @ gps's 101-02 kph. then sit back and pass everyone doing "100" but yeah i keep a sharp eye on it and press decel if it starts hitting 103...it feels naughty but it's perfectly legal, ive driven past a few cops with their guns out and nothing ever happens. i'm a cruise control addict

[tknova]

After going to 235/35/19's and running on the Tom Tom speedo is 100% spot on

100km/h on the speedo is 100km/h on the tom tom.