Speedo inaccurate ?????

Quote:

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Originally Posted by Ben-e-boy

There wasnt any other cars on the road and it was in central queensland

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That explains it QLD.
Na just playing.
sounds broken.
I've placed these things out for the RTA and they are calibrated to be fairly accurate.
Thats why I wrote such info.
Just trying to help.
cheers

As someone has previously said, speedos in cars are designed to over read the actual speed based on the manufacturers suggested rims and tyres for the vehicle.

I bought a GU4 last October with 265/75R/16s and was astonished when my GPS and the speedo were in perfect agreement. I replaced those tyres with 265/70/R16s and on my journey today with my GPS found that my speedo is now about 7%/8% out and when I'm traveling at 108 kph according to the speedo I'm actually doing 100kph according to my GPS. The 265/70/R16s are Nissan's recommended tyre size according to the label in my glove box. What made the difference was tyre height.

speed difference will even change with tyre wear,

Tim

Update...
New, 'factory spec', highway tread tyres fitted.
pumped to 40PSI.
Speedo still ~5% high.
Gearbox take-off gear is next on my hit list.

Thanks for all the great info.

Dave.

Mine runs at exactly 10% over. Use a GPS quite regularly and the speedo overstates how fast I'm going. It doesn't bother me cause I know it's pretty unlikely I'll ever get pinged.

Chris

My speedo is 100% correct. Tested it with 3 seperate GPS units and the speedo always matches the GPS reading.
I think they should all have to be 100% correct. Everybody knows that they are deliberately overstated so they compensate for it by doing say 105 in a 100 zone. However their speedo may only be out by 3kms for all they know so they are speeding. If they were all 100% correct you would do 100 as it shows on the speedo.

You have a very unusual speedo, cobber. Vehicle speedos are rarely linear. It costs waaay too much to even attempt to make them that way. It just means you have lucked out with that 1 in 1000 instrument.

It is well nigh impossible to calibrate any driven sensor to be 100% accurate without huge dollars.
Tyre size, tyre wear, winter, summer, inflation pressure, road surface, vehicle load plus about a page of other variables will all affect the reading.
Even changing the brand of tyres in exactly the same case sizes will change the error. EG Cooper 33's are closer to 32's IE the circumference of a Cooper 33 is different to a Procomp is different to a BFG is different to a Mickey T etc.

I don't use my speedo at all and just use one of the GPS readouts. Works for some people but not others...

The whole issue is a technical one. Most if not all electronic speedo senders count number pulses at the dedicated "pole wheel" either at the prop shaft or driveshaft. Things which influence the pulses most are you've guessed it final drive ratio and tyres. The unpredictable variable is a tyre size as it is dynamic, it changes as tyre wears, the load imposed and its inflation pressure because all these factor influence the rolling radius which in turn influences how many times the tyre turns per distance. Number of turns is than read as impulses per unit of time aka frequency ....yadda, yadda you'll get a point.
The more impulses per distance the faster the speed reads out. As the tyre is wearing out, or is compressed by load and less/more inflation pressure the more impulses are read hence the speed is read faster even though the actual speed is less. So technically fitting brand new tyres which are e.g. 3 % bigger than worn out ones would cause a difference of 3% at indicated 100km/h so actual speed would be 103 km/h if it was exact/accurate when tyres are worn by 3 % as an example. This also influences odometers, fuel trip meters etc.
To combat/avoid this the deliberate error is factored in so the less read speed than actual speed never occurs.
This tyre dynamic characteristics can be actually advocated/used as an "el cheapo" tyre pressure monitoring on ABS/EBS equipped vehicles where control units are able to sense the difference in speed among wheels and deduct that a particular tyre is/might be deflating and warn a driver.
Cheers

Ok my mate drives a TJ wrangler with 33" claws I drive a troll with 33" mtz's he's auto I'm manual I think both came standard with 31's his speedo reads around 90km at 100km and mine 110km at 100km checked at the same time same road same gps?????


Tappin N Crappin

When you put larger tyres on it decreases your speedo. So 100km/h on the speedo is actually a bit higher than that. So yours sounds right but his seems like a strange one.