Hi all,
Got a question about the 380 speedo...
Story in brief, on the way to work this morning I passed a cop van (like a Ford Transit or something) parked in the X lane in median strip on M4. Don't know what he was doing, whether speed checking one side of M4 or both sides.
I was overtaking a car, ironically a 3rd gen Magna with chrome rims. By my speedo I was doing ~112, but as I recall from my GPS (died months ago unfortunately) this speed in 380 is equal to 100 in real speed. So I'm guessing I was doing about 100 (give or take a 1k or 2).
So just wanted to confirm, is that right? 110km in 380 = about 100km real speed?

From memory, the last time I check with my VRX 17 inch rims I was doing around 106 when the speedo said I was doing 110.

When I last used the GPS, maybe about 6 months ago (nothing has changed with car, tyres, in that time), I noticed that I had to push the speedo a tad over 110 to show 100. To check the accuracy of this, I did it on my other car (a 1996 Commodore) and its speedo reading was spot-on.
EDIT: Just remembered something else... when I dynoed my car, I recall hit 220 on the speedo but my mechanic said his computer only showed 195. That was the first time I knew the speedo was out.

My VRX with stock 17s is about 7-8% fast. I.E. 108 on speedo is 100 on GPS 120=110. Of course if you have diff wheels & tyres you will be different again. Even two cars with same setup but different tyre wear will get different results.

:stoopid:

That was about my error on stock 17".

After going to 19" with 40 series, my error came back to 3kmh (checked by speed camera - with complimentary photo :( )

Have 40 series 17' and when i sit on 110 it is actually 100. This has been tested with GPS and also a number of mates cars. When i put the stock 16' back on it is within a couple of k's. My 17" are acually a smaller rolling dia then the stock 16"

19 in ROH wheels and tyres and I get +3 km/hr error.

Your 10k error is huge.

When I did my dyno run, I was still on my 16s. When I checked with the GPS, I had the 19s. Both times, the speedo reading has always been constantly the same (ie wrong) regardless of the tyres/wheels. So I guess no 2 380s are alike; I thought they were all the same or similar, but as that's not the case my original question is therefore voided.

i dont think 'my GPS said i was ok' would stand up if it comes to it :)

and u can always get done for having a faulty speedo too. :)

i dont think 'my GPS said i was ok' would stand up if it comes to it :)

and u can always get done for having a faulty speedo too. :)

Food for thought, but I'm confident I'll be okay.
I zipped past a hidden cop car on the M7 (couple of years ago) on a long weekend (so double points) doing 110 in 100 zone, but never got an infringement.

So far we have a few running 16s, a few with 17s and a couple with 19s. So just to fill in the gap, I run 18s.
235/40 ZR18s to be exact.

And my speedo over reads a lot.

Speedo 120km/h = true 113km/h according to two different satnavs.

So on the open road in a 110km/h zone I set the cruise to 120, Plod isn't (yet) going to ping anyone for 113 in a 110zone.

KJ.

Its not the manned BiB you have to worry about its those damn Vans and fixed cameras

Nice to know I'm not going crazy. Running stock VRX wheels and my satnav was way out like most here. I thought it was the satnav but now I'll trust it a bit more...

All car makers are making the speedos slower to avoid their customers having issues with revenue raising

ADR only needs them to 90% correct +/- over 100 kms

Too bad my corolla speedo is dead on accurate when I passed a BiB on the Kings Hwy @ 145km/h

So far we have a few running 16s, a few with 17s and a couple with 19s. So just to fill in the gap, I run 18s.
235/40 ZR18s to be exact.

And my speedo over reads a lot.

Speedo 120km/h = true 113km/h according to two different satnavs.

So on the open road in a 110km/h zone I set the cruise to 120, Plod isn't (yet) going to ping anyone for 113 in a 110zone.

KJ.

It appears the speedo accuracy has nothing to do with the size of rims, as there is everything from 16s to 22s here - and we're seeing readings over and under. I guess each speedo is just the way it was made. So the way I figure it, I was either right on the limit or a max of 2 over. And I really doubt I'm gonna get busted for that... but if I do, its 1 point (thanks to new rules). I'm sure the van was there to catch the big fish, not the little ones.

17in on my Gt reading was actual 99 when reading 100km/h

It appears the speedo accuracy has nothing to do with the size of rims, as there is everything from 16s to 22s here - and we're seeing readings over and under.
AFAIK it's more to do with the fact when you change wheels you change the diameter even if it appears matched up it will change +/- a handful of mm and will will have an effect.

Once you know what your own wheels are out in % terms. Then you can go from there.

I so dont want the change my stockies anymore

I am happy with the running slow

I was just thinking about all these different rolling diameters changing the speedo readout & wondering does this also affect dyno readouts & if so by how much & in which direction?? :hmm:

I was just thinking about all these different rolling diameters changing the speedo readout & wondering does this also affect dyno readouts & if so by how much & in which direction?? :hmm:

I don't think it makes any difference, in that the dyno readout adjusts for tyre size (and hence rolling diameter).

When Steve Knight set my car up on his dyno one of the first parameters he keyed in was my tyre size.

KJ.

I don't think it makes any difference, in that the dyno readout adjusts for tyre size (and hence rolling diameter).

When Steve Knight set my car up on his dyno one of the first parameters he keyed in was my tyre size.

KJ.

Ok well I guess gearing is taken into account as well then?

Ok well I guess gearing is taken into account as well then?

Yep, Steve also keyed in my 3rd gear ratio (coz that is the gear the runs were done in) & final drive ratio.

KJ.

Its not the manned BiB you have to worry about its those damn Vans and fixed cameras
If you guys haven't noticed at all lately, there hasn't been any camera cars around for ages.

All car makers are making the speedos slower to avoid their customers having issues with revenue raising

ADR only needs them to 90% correct +/- over 100 kms

Too bad my corolla speedo is dead on accurate when I passed a BiB on the Kings Hwy @ 145km/h

145+ was that an on the spot loss of licence? :pray:

145+ was that an on the spot loss of licence? :pray:

Nope pulled me over gave me the third degree saw my wife Pissed off so bad decided to give me a ticket for 15 and under

He was a good bloke! I pulled over as soon as he turned his lights on and I treated him with respect and had a yarn about conditions along the road where I was going any better routes and just basically remained cool

I knew I messed up so I was not going to be a prat

I have much respect for Hwy Patrol (does not help that 4 of my mate are HWY patrol men)