If smaller tyres make the speedo read faster than you're actually going then you'll never get a speeding ticket, will you?
Whatever bit of legislation you think your dealing with, in the case of a vehicle being sold with under-sized wheels you would still have to show evidence that the seller a) knew the tyres were under-size and b) understood the effect that would have on the speedometer in order to win a compensation claim.
I appreciate that, it's just that the OP wasn't specific and we've all been assuming he meant diameter (I think).
No but that's not the point
Don't agree. Ignorance is no defence.
Ignorance of the law is no defence but that's not the same as being ignorant of the fact that the wrong tyres are fitted.
On or around Tue, 14 Aug 2007 17:40:45 GMT, SteveG enlightened us thusly:
The speedo is allowed to read over by 10%, i.e. it can read up to 33 at 30. It's not allowed to read 27 at 30.
However, it'll only under-read if the tyres are over-sized, if they're under-sized, it'll read high, so that's a non-starter from the POV of "wrong size tyres".
On or around Tue, 14 Aug 2007 17:43:46 GMT, SteveG enlightened us thusly:
IIRC, he was sold a LR which should be on 7.50x16 or 235/85R16 or such and it was on something like 195R16. Which is undersized, but I reckon you'd have a hard time proving that this made it unsafe.
One possibility is the weight capacity of the undersized tyres - if they have a maximum load marking, you might be able to get 'em on the fact that they're not up to carrying the max weight of the vehicle.
In news: snipped-for-privacy@4ax.com, Austin Shackles wibbled :
However ALL of that tolerance can and is attributable to the speedo head, ie there is no residual tolerance for the rest of the system. Oversize tyres may and probably will cause the speedometer to read low. Thus illegal!
If you don't like my sums feel free to invent your own, in particular how to spread the overal tolerance throughout the system. You can't, each part has to accomodate the limiting cases.
Hello, Stuart! You wrote on Sat, 11 Aug 2007 15:14:16 +0200:
SG> Does anyone know offhand where I can find the relevant law which SG> prohibits fitting tyres which are smaller than original equipment? The SG> "the Land-Rover you sold me is crap" case is coming up in front of the SG> small claim court.
SG> Stuart
Well .. thinking on this, the only way you would get them is if either (or both)
I'm pretty sure that "size" is irrelevent (for once!)
So ... what are the markings on the tyres?
The -0% +10% spec is a dishonest piece of law. An engineer can never do 0% so he reads that as over read by 5% then +/-5%
Hence all speedometers are inaccurate. By law.
nigelH
Absolutely, no argument from me on that :-)
SG: The fitment of the over (or under) sized tyres isn't illegal they just cause an illegal effect. Using that reasoning all cars would be illegal because they have engines which enable them to break the speed limit. You can overcome the reading error by using a speedo with the correct gearing ratios. Land Rover used to make several different speedo heads with different gearing.
It's not obvious to me that 100% of the error in a speedo is attributable to the instrument itself (if that's what you mean by the head). Surely the transponder mechanism in electronic systems and mechanical drive in cable driven systems will have some error/degree of uncertainty? It may be small but it can't be ignored, surely?
SG: Sorry, haven't seen the arithmetic so can't comment on it's accuracy/validity.
Cheers for that Austin. If it comes down to purely a question of "wrong size" ~ all other aspects being okay ~ then I personally think the OP is gonna lose.
A friend of mine has a copy of Hughes Guide To Traffic Law. I'll give him a call tomorrow and see if there's anything in that.
Being totally facetious, do they make a tyre that has a speed rating lower than the top speed of a Landie?
Only ones to match the recommended tyre sizes
A rotation is a rotation. The only variable outside the speedo head is the manufacturing tolerance on the tyre and the correct inflation pressure.
Yep, Greenway Anaconda, rated to 70MPH! Got them on the pinz (max speed 60MPH)
Hi Ian, I was thinking of Series Landies when I wrote that :-)
That's far too simplistic. So how come they can manage to manufacture speedo drives without any tolerances but not the wheels and tyres? EVERYTHING has a manufacturing tolerance and, by default, some inherent degree of uncertainty/repeatability from one cycle to another.
In an electronic speedo drive you have both mechanical and electronic components. The pick-up itself is electro-mechanical and it's repeatability will be effected by the speed of rotation of the trigger and the response time of the detector (which will, in turn, vary with temperature and vibration). This is a potential source of error in the reading at the speedo head.
With purely mechanical drives you have backlash in the worm gear, drag in the cable (which varies with speed and temperature) and wear in the interfaces between cable and worm gear/speedo head. Again, these have the potential to cause an inaccurate speedo reading.
Oh right! In that case, probably not ;-)
Simex trekkers are rated to 50kmh IIRC
-- "For those who are missing Blair - aim more carefully."
To reply direct rot13 me
bURRt the 101 Camper
Okay, I put my hands up :-)
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.