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JimboGQ
5th October 2010, 11:34 PM
Hi guys, this is my first time on a forum, not sure if this is the first time this question has been asked but I have 285/75/16's on a GQ 4.2 carby and at 100km will my speedo be out and if so roughly by how much? I drive on the freeways at 95km and all cars fly past me but I don't want to risk going over the speed limit as in WA the coppers have cameras everywhere over and have numerous fines (have a heavy right foot which I need to lighten)? Any advice would be greatly appreciated.
Cheers, Jimmy

rkinsey
6th October 2010, 02:22 AM
Hi Jimmy,

I would have the speedo calibrated as there could be several reasons why your speedo could be out. I did a search on google and came up with these guys http://www.ahgwa.com.au/Duncan_Nissan/departments.php?dept=32

Cheers,

Rob

DX grunt
6th October 2010, 06:54 AM
Hi Jimmy.

Welcome to the forum. From memory, there's a place called Australian Instrument Services (AIS) in Perth somewhere, that re-calibrates speedos.

Take care out there.

Ross
DX grunt

Edit: May be Allied Instrument Services or Aust Instr Svcs. can't remember.

Bigrig
6th October 2010, 08:07 AM
Hey Jimmy

same goes for me - have a set of 285/75/16's and when I sit behind my mates brand new BT50 with him on 100, I am only doing about 94 (ish) ... not sure of WA, but in QLD now it is illegal to have ANY level of over speed calibration on the speedo itself (there was an ADR that allowed for up to 10% either way originally, but this has now been canned for the new guidelines) - i.e. not only can you get done for speeding, but if you mention you were only doing 100 to the boys in blue, they can order a mechanical and ping you for the over estimation on the speedo calibration also ... definitely worth just taking it to one of the joints mentioned above for peace of mind - if they are anything like here in QLD, they will under calibrate by about 2-3% (i.e. when you are sitting on 100, you'll actually be doing about 98 etc).

Good luck with it ... they are cracking down on this everywhere by the sounds of things ...

JimboGQ
6th October 2010, 12:21 PM
Thanks guys for the replies and definately will get it looked at.

Cheers again, Jimmy

kopyme
9th November 2010, 01:19 PM
Hey Jimmy,

Just to let you know that Marks 4wd does a speedo calibration electronic box to fix the problem too.

JimboGQ
25th November 2010, 07:43 PM
Thank's kopyme I have just ordered a calibration box as it will be quicker for me to order online and fit on a Sunday than getting time off work. I was going to get the old girl calibrated over the Christmas break but this will work out a lot better. Cheers again to all the above for their imput.

Jimmy

sweet_gq
26th November 2010, 07:48 AM
I have the same problem, I run 285/75/16's...

Lined up my speedo, odometre against my GPS and found about a 5% variance. I haven't worried about the marks adaptor. Just keep it in mind when travelling.

B

Finly Owner
27th November 2010, 12:15 AM
Try this to explain tyre v speedo

http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx

tkn
1st December 2010, 12:37 PM
All cars have "error" between speedo reading and actual speed. If the error is about 5% or less, then best to live with it. If you can't, then buy tyres with 5% larger rolling diameter - you can check this by putting a white spot on your current tyre and mark the ground, then the vehicle forward until the tyre rotates 360 degrees and measure the distance travelled. Your proposed new tyre should be 5% more - don't take all said in tyre brochures as gospel. The travelled distance does not have to be millimetre perfect, an error of say 3mm in 2.4 metres will be negligible.
You say you have a "lead foot" - use a bit of reverse phsychology on yourself, leave the speedo as it is and you will find that you will drive to 100Km shown on the speedo (or a bit over), keeping you within the speed limit.

Matmobile
8th December 2010, 12:06 AM
if you can borrow a gps they usually have the speed on them and you can check for certain

tkn
5th January 2011, 10:26 AM
I installed the GPS over Christmas - only been sitting the box for 6 months. There was a bit of a surprise in the gps vs speedo figures - mainly because there is no consistent error in the speedo i.e. not 5% for all speeds. So I'll list a few of them -

GPS = 38 / speedo = 40
GPS = 60 / speedo = 63
GPS = 80 / speedo = 85
GPS = 90 / speedo = 99
GPS = 100 / speedo = 111

So in school zones the speedo is within 5% and on the highway it is more than 10% over reading the actual (GPS) speed.

the evil twin
5th January 2011, 11:07 AM
I installed the GPS over Christmas - only been sitting the box for 6 months. There was a bit of a surprise in the gps vs speedo figures - mainly because there is no consistent error in the speedo i.e. not 5% for all speeds. So I'll list a few of them -

GPS = 38 / speedo = 40
GPS = 60 / speedo = 63
GPS = 80 / speedo = 85
GPS = 90 / speedo = 99
GPS = 100 / speedo = 111

So in school zones the speedo is within 5% and on the highway it is more than 10% over reading the actual (GPS) speed.

Yeah, its at the high end but thats pretty much in the standard ballpark for speedo error with 31's.

ADR's state the speedo cannot underread by even 1 KPH but can overread by up to 10% of actual speed plus 4 KPH so at 100 actual vehicle speed your speedo must indicate between 100 and 114 to comply with ADR's. Google it if you wish

If you want to "calibrate" the speedo you can buy replacement drive cogs ( I think they have 14,15,16 and 17 teeth options and are colour coded) IF you aren't at the low end already. These are actually to allow for different diff ratios between models EG 3.9 or 4.1 diffs but will work fine to adjust the error. Pretty easy to swap.

Usually if you go up 2 inches in tyre size or swap the cogs the speedo ends up darn close to accurate but throws the odometer out...

Don't make the same mistake many do and assume that if the speedo is 10% out so is the Odometer... it might be in some cases but very often it will be different so check the Odometer against either the GPS as well or known mileage pegs. This is one of the reasons some people get significantly different fuel figures and indeed the Speedo in my current truck with 33's is almost spot on but the Odo undereads by 8 K's in 100

NissanGQ4.2
5th January 2011, 02:32 PM
how accurate are GPS speed readings????

the evil twin
5th January 2011, 05:11 PM
how accurate are GPS speed readings????

Extremely. IE if your speed is stable and the GPS says your doing 100 KPH then you are all but guaranteed to be doing between 99.5 and 100.5.

There is one proviso to that statement, if it is a crappy 15 minute window where a low number of Sats are visible or you are twisting and turning thru the CBD it is possible to get timing and multipathing errors so check it a couple of times but if more than 5 or 6 Sats are visible for the GPS to choose and get an accurate 3D fix and you are cruising down an arterial, highway or freeway it will be spot on 99% of the time.

Easy way to tell is...
If you have a true GPS Reciever (Garmin, Magellan, Trimble etc) check your "LDOP" or "Accuracy" dpending on maker they will usually use one of those terms. If it is 5 metres or so then all is good.

If you are using a PND or PDA (Tomtom, Navman, Asus etc etc) look at your "Satellites" "GPS" or "Skyview" screen and if there are 5 or more pretty evenly distributed within the circle, then again, it will be very accurate

mackayboi
19th January 2011, 03:20 PM
I suppose you can always get a friend to travel with you on the highway and one waves his/her hand to inform you that they are travelling at 100KM/H.
The test car and the patrol should both be in par with each other...
The test car could preferably be a new car with electronic speed readout...

Just my two bobs worth...

YNOT
19th January 2011, 03:30 PM
Even new cars have inaccurate speedo's.

Tony

the evil twin
19th January 2011, 07:13 PM
Even new cars have inaccurate speedo's.

Tony

Exactly... new or older, the Speedo calibration has to be exactly the same as everyone elses to pass ADR's (10% of actual plus 4 KPH)... even a GQ Speedo is "Electronic"...

moo_r32
26th April 2011, 11:48 AM
i had a simular problem, all i did is chuck my car on the dyno at my work and made note of the common speeds.. eg 60, 80 and 100 kph.. iv got 35's and 60K's on my dash was 65 and 100 kph on my dash was 108 on the road..