I have tested my speedo against a radar gun with a large digital display atached to it, You usually find these radars parked outside schools to promped you to slow down. My speedo reads 80km but the radar reads 74km, With my factory wheels and tyres on too. Has anyone else noticed this ??? Thats not even close to accurate:doubt:

Yeah I've seen those school-side/roadworks displays; they're always wrong.
As for the 380 speedo, mine's inaccurate too. Using my GPS to hit 100kph, the 380 speedo is on around 110 (yet my 1996 Holden Berlina's speedo is 100% accurate).
Look on the bright side, if you follow your speedo all the time you won't ever get caught speeding.

They tune them to read faster than you're travelling, within the 10% allowance. This is to ensure there is less incidence of speeding due to speedo inaccuracy. It's a safety consideration.

yeah those "advisory speed checks" on the highways always say my speedo is about 3% out (speedo says 100, speed is 97) and gps confirms it.

Now my bike is pretty far out. 70 on the cars speedo is 74 on the bikes. So there must be another 3% on that behind.

GPS isn't instantaneous and isn't always accurate due to the changes in altitude - as in the rise and fall of a road surface.

I cant help but dwell on the fact that this inaccuracy will make the 380 look alot more economical then it really is. Was this done on purpase to give the 380 a lower fuel usage per 100km in order to sell more cars? I feel in this day and age there is no excuse for for this, I smell a rat:rant:

actually you'll find most speedo's are out by the same ammount, if you sent your car's cruise controll by the speedo you will find your doing the same speed as most other cars out there, if you set it via GPS or whatever then you'll be going about 10k's faster than everyone else.

I tend to think it's a safety thing.

I cant help but dwell on the fact that this inaccuracy will make the 380 look alot more economical then it really is. Was this done on purpase to give the 380 a lower fuel usage per 100km in order to sell more cars? I feel in this day and age there is no excuse for for this, I smell a rat:rant:

Switch to aftermarket 19" wheels and you too will gather speeding fines :P

After I got my wheels I set my cruise on ten percent over, as usual, and was shocked when I got my first fine. The car now runs within about 2% of the speedo readout.

GPS isn't instantaneous and isn't always accurate due to the changes in altitude - as in the rise and fall of a road surface.

nevertheless, GPS will be more accurate to the true speed than the speedometer will.

GPS being accurate to calculate speed? No.

Over a large distance, yes. On the fly, no. Your GPS is accurate (depending on model/weather/whatever) to within 2-3m.

It determines your speed by dividing distance by time. If its out a few metres here, a couple there, its out by a fair bit for on the fly.

You can also have your speedo calibrated if you think its out, and should be done when you change wheels.

GPS being accurate to calculate speed? No.

Over a large distance, yes. On the fly, no. Your GPS is accurate (depending on model/weather/whatever) to within 2-3m.

It determines your speed by dividing distance by time. If its out a few metres here, a couple there, its out by a fair bit for on the fly.

You can also have your speedo calibrated if you think its out, and should be done when you change wheels.

I didn't say it was tottally accurate, just more accurate to true speed than the speedometer

GPS being accurate to calculate speed? No.

Over a large distance, yes. On the fly, no. Your GPS is accurate (depending on model/weather/whatever) to within 2-3m.

It determines your speed by dividing distance by time. If its out a few metres here, a couple there, its out by a fair bit for on the fly.

You can also have your speedo calibrated if you think its out, and should be done when you change wheels.
Well, Bigger or smaller wheels anyway.

GPS being accurate to calculate speed? No.

Over a large distance, yes. On the fly, no. Your GPS is accurate (depending on model/weather/whatever) to within 2-3m.

It determines your speed by dividing distance by time. If its out a few metres here, a couple there, its out by a fair bit for on the fly.

You can also have your speedo calibrated if you think its out, and should be done when you change wheels.
I went from 16s to 19s on my car and speedo is still the same as it was before. Maintaining the same overall diameter of new wheels and tyres won't effect your speedo.

GPS should be accurate to about .2-.5kmph. Just make sure your not looking at it when your accelerating and you will be fine.

So.. basically, I can set the cruise on the motorway, cruise for a while on 100 (speedo) and check the gps to see how much higher/lower the speedo is?

If I were to do this, and work it out in my head driving along regularly, would I take x km/h off of the speedo readout to get an accurate speed, or would I take a percentage off the speed?..

The oldschool way is mile markers.

There should be speed check signs on a highway near you.

It's spread over 5km, with a sign every 1 km, and a warning sign 1 km before the first.

Get a stop watch, time how long it takes you to do the 5km.

I dont get all these fancy gadgets......

once you work out how far out your speedo is, you should apply that percentage across the whole speed range.

i.e. if your speedo reads 105km/h and your GPS reads 100km/h, then your speedo is reading 5% high. so at 84km/h indicated you're travelling at 80km/h and so on.

VW have admitted that they have the speedo PURPOSELY read 6-7% high. however the speed sensor is bang on, and the odometer/trip computer is run off that.

with my Magna EVERYTHING out by about 4.5% (odometer/trip computer/speedo) so the odometer is reading more km's than it's actually done, and the L/100km is off by the same percentage!

of course, I'd prefer everything to read correctly (they have the technology!!) but if they had to, I think I'd prefer just the speedo to read high, but everything else be correct.

cheers,
.wook

So.. basically, I can set the cruise on the motorway, cruise for a while on 100 (speedo) and check the gps to see how much higher/lower the speedo is?

If I were to do this, and work it out in my head driving along regularly, would I take x km/h off of the speedo readout to get an accurate speed, or would I take a percentage off the speed?..
Even better, have your passenger read out the GPS speed as you tune into 100Km/h by GPS, then at that cruise speed, check your speedo error. Mine indicates 106Km/k on the speedo for 100Km/h by GPS.

Next time I'm due for tyres, I'm considering upgrading to slightly wider tyres with same profile, so the circumference will be a little larger, and the speedo error lower.

If you are really worried, just get it calibrated, costs ~ $100.

Orge - wider tyres wont change the overall rolling diameter, so you're speedo wont change.
if he stays with the same profile it should. but not by much. and depending on the tyre brand the same size might be +/- 5% anyway.


If you are really worried, just get it calibrated, costs ~ $100.
or get one of these (http://www.jaycar.com.au/productView.asp?ID=KC5435)...

cheers,
.wook

Wookie - maybe im missing something here :confused:

He wants wider tyres, say from 7" to 8" wide. Why would this change the rolling diameter if he stays with the same profile/size of wheel? :confused:

Wookie - maybe im missing something here :confused:

He wants wider tyres, say from 7" to 8" wide. Why would this change the rolling diameter if he stays with the same profile/size of wheel? :confused:

because the sidewall height is a percentage of the tread width.

like I said, it won't be much. so 245/40r18 have a larger overall diameter than 235/40r18 (645.2mm to 653.2mm). it's only 8mm difference, but it is larger.

cheers,
.wook

Wookie - maybe im missing something here :confused:

He wants wider tyres, say from 7" to 8" wide. Why would this change the rolling diameter if he stays with the same profile/size of wheel? :confused:
Okay guys, the profile of a tyre is expressed as a PERCENTAGE of the section width.

Eg 205/60R15 means the profile height is 60% of 205mm. The profile height of 215/60R15 would therefore be greater.
Does that explain it better or do you need to see circumference calculations?

Orge - wider tyres wont change the overall rolling diameter, so you're speedo wont change.

If you are really worried, just get it calibrated, costs ~ $100.
Nah, I'm not really worried, but passengers are when I forget to explain my "apparent" speeding to them!:badgrin:

Ah as a percentage - sweet, learnt something new thanks guys :D

Even better, have your passenger read out the GPS speed as you tune into 100Km/h by GPS, then at that cruise speed, check your speedo error. Mine indicates 106Km/k on the speedo for 100Km/h by GPS.

Next time I'm due for tyres, I'm considering upgrading to slightly wider tyres with same profile, so the circumference will be a little larger, and the speedo error lower.

So.. if mine says 106km/h when I'm actually doing 100, when i'm doing 50, would the speedo say 56, or 53?
Thats what I meant, is it a percentage out, or is it just a certain number of k's out?

Thanks :D

Its a percentage.

It would be 53km/h.

So.. if mine says 106km/h when I'm actually doing 100, when i'm doing 50, would the speedo say 56, or 53?
Thats what I meant, is it a percentage out, or is it just a certain number of k's out?

Thanks :D
In my example, I have a 6% over-read, but no offset error (When I'm stopped the speedo is at zero). As you scale this 6% down, 53 on the speedo is 50 by GPS. It is safe to assume a certain degree of linearity with the speedo error.

Yeah, I complained to mitsi about my 4-5% error when I got my car and they said it was within their 5% limitation. Wasn't too happy with it, but it was bearable. Changed my rims to 18's from the stock 17's and now it's 8% off (pics to come). I'm going to make them change my speedo when I get it serviced - $100 is a bit expensive but I guess it's well worth it.

Checked my speed on the highway with GPS. IMHO GPS provides absolute position every 1 sec. This means it's accurate as a speedo as long as you stay with the same satellites and maintain the same velocity (ie no acceleration/deceleration/turning) within that second. On an open country highway there should me minimal interference and so it is likely that the error values will remain the same - you don't need an accurate position, only an accurate measurement of the difference between calculated positions in time.

Yes, l took a trip out of town and my GPS said l was travelling 6km less than my speedo. Not sure how accurate GPS is.

Yes, l took a trip out of town and my GPS said l was travelling 6km less than my speedo. Not sure how accurate GPS is.



By my experience, comparing the speedo reading with a satnav reading, the 380 speedo is optimistic (over-reads) by about 6%.
So when the speedo reads 110 km/h you're really doing only around 104 km/h. On stock tyres.
IIRC, since 1999 under the Australian Design Rules (ADRs), speedos must never under-read and they can over-read up to 10%.
So the 380 speedo was deliberately engineered to over-read by about 6%.

By my experience, comparing the speedo reading with a satnav reading, the 380 speedo is optimistic (over-reads) by about 6%.
So when the speedo reads 110 km/h you're really doing only around 104 km/h. On stock tyres.
IIRC, since 1999 under the Australian Design Rules (ADRs), speedos must never under-read and they can over-read up to 10%.
So the 380 speedo was deliberately engineered to over-read by about 6%.



It is actually a bit more complex than that - the ADR says:


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).


More details and how to measure etc at http://www.comlaw.gov.au/Details/F2006L01392




0 £ (V1 - V2) £ (0.1 V2 + 4) km/h

1 Can't under-read as reading minus true must be greater than or equal to zero


2 At V2 (true speed) of 110 kmh it is allowed to read up to 0.1 x 110 plus 4 ie 15 kmh over