ON THE WAY: BMW F21 M140i
MERCEDES GL350 - Metallic Navy Blue w/Grey interior

GOING: BMW F31 335D MSport+ xDrive Touring
GONE: Land Rover Discovery 4 HSE - Stornaway Grey w/Black Interior (wish we'd kept it )
GONE: BMW F10 520D MSport - Carbon Black
GONE:MY12 RRS SDV6 HSE Sumatra/Ivory/Grand Black Privacy glass

As the speedometer reading is calculated from wheel rotation how can it always be accurate? Considering the tyres can loose 6mm from new to still legal the distance travelled varies greatly per measured revolution.

Of course you could try steel rimmed wheels like a train, or throwing out a concrete block with a long knotted rope attached and check time against number of knots flying through your hand. Both somewhat impractical so what we have does a fairly good job at what they do, bearing in mind speed limits are as their name suggests maximum limits to remain under and not targets to aim for.

Exactly - tyre wear is part of the tolerance built in to the allowed inaccuracy. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Thanks for the tips.
Looking at be calculators, the 255/45 is the biggest recommended profile on a 20" rim.
I guess what's best to remember is that the profile figure is a percentage if the tyre width, so a 45 on a 275 is more rubber than a 45 on a 255.
I think I'll look around to see what looks good

Many many years ago in Australia, the RTA (road traffic authority) had cars driving around with large speedo's on their boot lids. When you see them, you could simply follow them and get an idea of the accuracy of you speedo. It was all too easy back then, now they prefer to collect revenue RRS '05 4.4 Bonatti Gray Black Leather
Discovery S1 3.9 manual with extractors
Mitsubishi Starion (ex group e production car)

Oops! On re-reading the manual, it states winter tyre size of 255/50/20 and not '45' in the first post

So;
Normal 275/40/20 gives profile of 110mm (4.33 in) (40% of 275)
Recommended winter 255/50/20 gives 127.5mm (5.01 in) (50% of 255)

Normal diameter = 20"+8.66"= 28.66"
Winter diameter = 20"+10" = 30"

Circumference = Pi x d
norm = 89.99"
Winter = 94.2"

1 mile is 63,360 inches;
In a mile a normal tyre would rotate 704 times A winter tyre would rotate 673 times.
With the difference being 31 revs per mile, the winter tyres would travel 133 inches less per mile and therefore under read on the speedo by 0.02%

I think

Is it all really just a simple matter of how much rubber you can fit without it rubbing on the bodywork, rather than the technical side?

Or you just enter your tyre size and the computer resets...im sure that options in there somewhere (diagnostic kit not the bit you see) Mike

2014 Facelift Discovery