Tyre size legislation

And even the 1.6 litre Series 1 is claimed to go that fast!

JD

On or around Wed, 15 Aug 2007 10:15:58 +0100, "GbH" enlightened us thusly:

yeah, but the OP has a case about fitment of UNDER-sized tyres. which can never cause the speedo to under-read if it was correct originally.

I know about this - Edward currently has an SWB speedo, designed for use on a series with 6.00 tyres, and since he's an LWB on 7.50 tyres it under-reads. If I fitted undersized tyres (e.g. 205R16) to Edward, the speedo would read correctly. There's no combination of OE speedo to my knowledge on a landy that will under-read without fitting over-sized tyres.

I wasn't arguing that over-sized tyres will make the speedo under-read.

On or around Wed, 15 Aug 2007 21:55:05 GMT, SteveG enlightened us thusly:

So have I. fantastically useful, on occasions, and about 1/100 of the price of Sweet and Maxwell.

On or around Thu, 16 Aug 2007 08:15:49 GMT, SteveG enlightened us thusly:

I had my series II (standard 2¼) up to about 80 on the clock once.

regularly got 70 on the clock racing trains along the A48 in Port Talbot... ah, happy days...

This is plainly false! We all know that the air is sucked out of your lungs at speeds higher than 30MPH. That was what common sense said back in the victorian era ;-)

Sorry to disillusion you Ian, but when I was young and stupid and didn't believe in such statements I saw the upside of 70mph in my dad's Series II and still appear to be alive (and somewhat wiser with the passing of time).

Hmm, did you read what I wrote?

I did actually refer to errors outside the speedo, not the speedo itself, and much of what you are referring to is transitional whereas we are more concerned with simple steady state.

However as you are obviously so knowledgeable about speedos can you explain something about the design. Normally tolerances are symmetrical i.e. + or - %age. but in this case the law requires a tolerance of zero in one direction and allows 10% in the other. Is the speedo system as a whole designed to match this skew e.g. to read 2% fast with tolerance of

-2% - +8% or is it designed to be 5% fast with tolerance of +/- 5%

By fast I mean giving a reading which is greater than the actual speed. Given the tyre sizes that have been fitted could this not well push the accuracy outside the design limit of the speed indication system?

What I don't seem to have to hand is the particular year and model involved and this would impact on the comparison with what LR actually recommend. Bear in mind also that tyre size doesn't just relate to speedo and load but also braking and suspension.

Going back to the OP I think he has a number of avenues to pursue now.

Type approval C&U Regs Land Rover themselves for a view. Impact on speed indication system Weight rating of the tyres compared to max permitted axle weight.

Well you did mention 'common sense' in that ambiguous statement, and we all know common sense is right, so you've only yourself to blame. LOL.

Martin

Hugh, it was 25 years ago that I worked for Smiths and things have changed a little since then. I don't really want to bump heads with you over this :-) In fact, this will be my last post on the subject.

In engineering (both mechanical and electrical) asymmetric (? I thought it had two s's but the computer says NO!) tolerances are very commonplace and it's relatively easy to cope with that at the design stage. Take a look at our pet subject here, the car speedo, for example; many of them don't start at zero - 10mph is quite common. The design engineer simply adds sufficient inertia to the instrument such that the pointer doesn't start moving until the vehicle is doing (say) 5mph and as the graduations start at 10 it's already reading high - hey presto, jobs done! Yes, I know the error is more than the specified amount but the -0/+10% only applies over a specific speed range not the whole range of the instrument/vehicle.

In my day type testing of speedo accuracy, repeatability and hysteresis was done on the instrument itself, in isolation from whatever drive was to be used in the car. The reason for this was to conform to the C&U Regs and BS testing procedures in place at the time. Additionally, it was quite normal to sell a specific instrument to several car manufacturers over time and the drive mechanisms could vary a fair bit.

Going back to the OP's original question; I still don't think there's a law which says you can't fit non-standard wheel and tyre sizes to a car. I know it may not be sensible and can, in certain circumstances, be bl00dy dangerous but I've yet to be convinced it's illegal.

There you go. Quite a lot for tuppence :-)