What speed am I doing with larger tyres

Submitted: Tuesday, May 16, 2006 at 13:21
ThreadID: 33939 Views:3223 Replies:13 FollowUps:12
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Hoping someone can help me with this. We have a 105 series landcruiser that we bought that was fitted already with larger tyres. It has 285/ 75R16 on it. Anyway couple of weeks ago the other half got flashed but a multanova (dirty thieving revenue raising device) in a 100 zone, she said she was absolutely certain the speedo said 100 or just under, anyway fine arrived saying she was doing 110. My question is would the speedo be 10% out by having the 285 tyres on it? Dont have a GPS to check it with.

thanks...
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Reply By: Member - Nutter (QLD) - Tuesday, May 16, 2006 at 13:27

Tuesday, May 16, 2006 at 13:27
Gidday Zapper , your speedo could quite easily be out 10%, think the LC 105 came out fitted standard with 265 x 70 r 16, 285s are about 10% larger rolling diameter.
I am pretty sure you can have it corrected, I'm sure someone on this forum will know, or just ahve a search.

See ya
Guy
AnswerID: 172926

Follow Up By: GaryInOz (Vic) - Wednesday, May 17, 2006 at 00:36

Wednesday, May 17, 2006 at 00:36
775mm vs 830mm or about 7% larger tyre, theoretically, however the ACTUAL tyre diameters are what you need. Given most speedos are a couple of km/h under (reads 100, is 98), you may well have been done by the tax collecters at 104-105.

I would question your ticket or your wife, one of them ain't telling the truth....
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Reply By: Member - Colin (WA) - Tuesday, May 16, 2006 at 13:30

Tuesday, May 16, 2006 at 13:30
G'Day Zapper'

I have the same size tyres on a 100 series and at 100 km/h you will be doing 105 km/h. We checked this by driving through one of those roadside speed indicators. The other way to check is by using a GPS .
regards Col
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Follow Up By: Member - Andrew W (SA) - Tuesday, May 16, 2006 at 14:38

Tuesday, May 16, 2006 at 14:38
absolutely. My exact experience with the same tyres/vehicle.

See here too: Tyre Size Calculator

Ciao for now
Andrew.
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Follow Up By: V8Diesel - Tuesday, May 16, 2006 at 15:17

Tuesday, May 16, 2006 at 15:17
I agree too. On my 105 I find when I have the splits on with 235/85/16 tyres the speedo is spot on according to my GPS. With the 285/75/16's it is out by about 6 or 7kmh.

I tend to set my electric foot by the GPS when travelling and use the resume button for the rest of the drive.

Just out of interest, do you blokes find your Cruiser's tend to wander a bit at speed with the 285 rubber on? I never really noticed until switching to split rims recently and found a big improvement with the skinny tyres fitted. Don't know if it's as a result of the bigger rubber generally or just a poor tyre design (Bridgestone 693's).
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Follow Up By: Exploder - Tuesday, May 16, 2006 at 19:14

Tuesday, May 16, 2006 at 19:14
Hear is a Good one> According to the Your Speed thingo they (Police) had set up at the road works at 60 on the Speedo this thing told me I was doing 64, According to my GPS when I am doing 60 on the Speedo my speed is 57.8, Umm yeah I hope the Speed cameras are a little more accurate than that thing !!!!
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Follow Up By: Marn - Wednesday, May 17, 2006 at 02:06

Wednesday, May 17, 2006 at 02:06
G'day v8. While i do not drive a landcruiser i did find that my str navara wandered around the road a heap more after fitting mud terrain BFGs even to the point when i was on the highway i stopped to check that i didnt have a flat tyre! Used to it now though.
Cheers.
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Reply By: Member - John L G - Tuesday, May 16, 2006 at 13:31

Tuesday, May 16, 2006 at 13:31
Zapper,

Sorry to be the bearer of bad news but my son has those tyres on his 100 series and we have checked them against his gps and my normally shod LC and guess what, 7% error was the figure we came up with.

Haven't adjusted the speedo, but have slowed a tad.
AnswerID: 172929

Reply By: Truckster (Vic) - Tuesday, May 16, 2006 at 13:35

Tuesday, May 16, 2006 at 13:35
Every car is diffferent.. Mass produced speedo's cant all be accurate.

Buy a GPS, you know it will come in handy for other things!
AnswerID: 172930

Reply By: Scoey (QLD) - Tuesday, May 16, 2006 at 14:01

Tuesday, May 16, 2006 at 14:01
Going from the standard 265/75-16's to 285/75-16's gave me a 3.7% difference on my 80 Series according to the Tire Size Calculator but I guess it all boils down to what the original tire size was, what the accuracy was originally like on the speedo and probably even the brand of tire you have - I've heard of two different brands of tire labelled as the same size actually being different in size.

Question: If you can only be found guilty of an offence to the degree that you are aware you are commiting the offence (ignorance of the law aside) and you buy a vehicle with a Safety Cert or RWC with larger than standard tires and aren't aware of this fact, is this grounds to appeal? They may make you get the speedo recalibrated instead which could be more exxy than the ticket but so might not be worth it! ;-)

Cheers
Scoey
AnswerID: 172934

Reply By: Gramps (NSW) - Tuesday, May 16, 2006 at 14:32

Tuesday, May 16, 2006 at 14:32
Zapper,

As some others have suggested, use a GPS to ascertain your true speed then either commit it to memory (I would'nt trust myself or someone else driving the vehicle) or get your speedo corrected or check out Jaycar or similar. I'm sure they have a device (adjustable) to fix/replace your speedo.
AnswerID: 172944

Follow Up By: Truckster (Vic) - Tuesday, May 16, 2006 at 14:46

Tuesday, May 16, 2006 at 14:46
Borrow a GPS, take white perm marker.

mark on speedo each speed
50 - 60 - 70 - 80 - 90 - 100 - 110
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Follow Up By: Gramps (NSW) - Tuesday, May 16, 2006 at 14:57

Tuesday, May 16, 2006 at 14:57
Come on Truckie. That's too sensible. Give Zapper a chance to have a gadget Roachie does'nt have ( I think ?????? )
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Reply By: blown4by - Tuesday, May 16, 2006 at 14:43

Tuesday, May 16, 2006 at 14:43
If you find out the revolutions per kilometre of the original tyres and the new ones you could then do a simple calculation to work out the new speed. The tyre stores have this info and it's on the tyres pamphlets as well.
AnswerID: 172947

Reply By: disco driver - Tuesday, May 16, 2006 at 14:44

Tuesday, May 16, 2006 at 14:44
Hi Zapper,
Are you aware that the ADR's for vehicles allow for a 10% variation in speedo accuracy when on new original tyres.
That is deemed acceptable by the various state authorities but the so called "Justice Depts" wont accept that as a valid reason if the "non-revenue raising speed cameras" take your picture.
Having said all that there are two ways of checking your speed.
1. A fairly long set of mathmatical calculations.
2. Buy or borrow a GPS and check the speedo against that.

Good Luck

Disco
AnswerID: 172948

Follow Up By: Jugs - Tuesday, May 16, 2006 at 15:05

Tuesday, May 16, 2006 at 15:05
Zapper

I have the 285/75/R16 on my GQ patrol Ti (on when bought same as you) I did the long Cal from the original 275/65/R16 and got 7%. you can get a after market plastic cog to correct the spedo sender. I haven't botherd just call it 10% accross the board for safty margin and subtract on the fly.
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Reply By: TerraFirma - Tuesday, May 16, 2006 at 15:42

Tuesday, May 16, 2006 at 15:42
The exact differences can be calculated at www.1010tires.com , click on tire size calculator. If the originals are 265x70x16 and the tires/wheels on the car now are 285x75x16 then the rolling diamter is now 6.8% larger which in turn would show the speedo reading 93.2KMh instead of 100. It puts the speedo out 7.2%.

Such an increase in rolling diameter is not recommended for various reasons.
AnswerID: 172955

Follow Up By: Truckster (Vic) - Tuesday, May 16, 2006 at 16:18

Tuesday, May 16, 2006 at 16:18
sounds simple, but some tires area almost an inch different measured from their marked size :(
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Follow Up By: Scoey (QLD) - Tuesday, May 16, 2006 at 16:22

Tuesday, May 16, 2006 at 16:22
There are also a lot of various reasons for increasing your rolling diameter by such an amount and more! :-D

Cheers
Scoey
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Follow Up By: blown4by - Wednesday, May 17, 2006 at 00:10

Wednesday, May 17, 2006 at 00:10
What like reduced braking ability for example?
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Reply By: 100TD - Tuesday, May 16, 2006 at 19:56

Tuesday, May 16, 2006 at 19:56
i have gone from std to 305/70/16 on my 100series and found it is out by 5kms(speedo says 100 and actually doing 105 by GPS and freeway speedo chk on western fwy!)i am shortly going to make a speedo correction box to correct this as better half constantly forgets that its not correct.cheers Paul
AnswerID: 173002

Reply By: Member - Bware (Tweed Valley) - Wednesday, May 17, 2006 at 02:56

Wednesday, May 17, 2006 at 02:56
Are you confused yet Zapper? If not, we haven't done our job properly ;)
If the GPS option isn't an option, then the tyre calculator already mentioned is a good guide. But..... there are all the other factors like massed produced speedos(also known as dick-stickers LOL), different diameters for different brands and also the diameter changes with wear (which hasn't been mentioned yet). If you use larger diameter tyres(which alot of us do) and are worried about speeding fines just drive 10kms less than the speed limit according to your speedo; that's the simple answer.
AnswerID: 173068

Reply By: TerraFirma - Wednesday, May 17, 2006 at 11:19

Wednesday, May 17, 2006 at 11:19
You can get your speedo re-calibrated various ways. When I bought my new 2005 Hilux they did the recalibration at the dealer via the box under the glove box. You can also drive past various speed check overheads, the one on the Hume for example I drive pass all the time. I have found my speedo to over-read my speed by around 4-5KMh at 100KMh which is a good thing.
AnswerID: 173115

Reply By: Member - andrew B (WA) - Wednesday, May 17, 2006 at 12:31

Wednesday, May 17, 2006 at 12:31
Speaking of Speed Camera's, the thriving metropolis of Kununurra has one lurking in the bushes now. A town like ours has a lot more serious problems which should be dealt with before our terrible accident record due to speeding.....like people sleeping on roads etc

Cheers Andrew
AnswerID: 173124

Follow Up By: GaryInOz (Vic) - Wednesday, May 17, 2006 at 17:41

Wednesday, May 17, 2006 at 17:41
If it is an unattended camera, give it some protection from the weather with a black plastic garbage bag zip tied over it. Or you could get an old unregistered bomb and dump it strategically to obscure the cameras view, and have another car in view of the camera (challengable in court due to radar reflections).
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