I was wondering how much of a difference to the speedo, odo, gearing and anything else technical, having a different profile size makes.

My standard fit tyres , on 20' rims are 275/40/20, looking in the handbook for winter tyre spec and it says 255/50/20. Does that increase of 5 make a difference and if not, how big a difference can the vehicle accept either +/- without it throwing a wobbly?

The standard tyre profile sometimes looks a bit too 'low profile' for the size of vehicle (I know it's a 'sport'), could I for example fit 50's with no side effects?

Thanks

Last edited by Tre4 on Tue Nov 06 2012 4:45pm. Edited 1 time in total

Google "tyre sizes" and you'll find a selection of tyre size calculating tools which will give all the info you are looking for.

The handbooks do have errors so common sense is required as Land Rover seem very reluctant to admit to or correct obvious typo mistakes.

I wouldn't worry too much about the speedo being out a little bit, you will find its already out by 3mph on your standard tyres @ 30 mph and out by 4 mph @ 70.

The car won't care what profile you fit just so long as you fit the same on each corner. The speedo / trip computer will read incorrectly but that's an issue for you, not the car  2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

You would be surprised how accurate the range rover speed limit is - it has the minimum required error (they all have to over-read so indicated 30 is a little less (10%) actual) Mike

2014 Facelift Discovery

So why can't they get it spot on if they can manage to get it to read 10% over

They don't have to over-read but they absolutely must not under-read.

As it's impossible to make sure they remain accurate for the entire life of the vehicle (unless, like some police vehicles, you have the speedo calibrated at regular intervals) the easiest answer to have the law allow an over-read tolerance and build them within that tolerance. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Surely then, they could make it part of the service schedule then or make it part of an mot test, Its not that hard to do, they manage to do it for HGV's. I know many police forces allow the speed limit + 10% + 2mph max.

RRSTDV8 wrote:

They don't have to over-read but they absolutely must not under-read.

As it's impossible to make sure they remain accurate for the entire life of the vehicle (unless, like some police vehicles, you have the speedo calibrated at regular intervals) the easiest answer to have the law allow an over-read tolerance and build them within that tolerance.

Ady 555 wrote:

Surely then, they could make it part of the service schedule then or make it part of an mot test, Its not that hard to do, they manage to do it for HGV's. I know many police forces allow the speed limit + 10% + 2mph max.

One word: money

Imagine the outcry if the Govt told motorists they'd have to pay for a speedo calibration once a year or every time they changed tyres which ever was sooner.

They system allows for the inherent tolerances in the tyre/speedo combination. If they asked for the speedo to be spot on and it could be showed to under reading at a court case then someone could get off a charge - or even whole groups of people would get off. So the system says the speedo must read between X and X + 10% where X is the indicated speed so for 60mph your speedo can read anything between 60 and 66 mph. To avoid erroneous court cases, everyone sets them up to be near the top of that range because that's the "safest" way to do it.

The ACPO guidelines for speeding offenses are just a way of limiting police work loads and preventing turning every driver in to a criminal... 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I suppose when you explain it like that, i guess it makes sense

with my new 20's my speedo is almost spot on (according to the radar speed display thingy in the village) RRS gone
Audi SQ5...Gone Audi Q7 S-Line 240 3.0TDI Gone
Audi A6 Allroad Sport 270 3.0TDI

^^^ so it reads 70mph as you'rebombing through the village, the same as your speedo  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

*just* 70?  RRS gone
Audi SQ5...Gone Audi Q7 S-Line 240 3.0TDI Gone
Audi A6 Allroad Sport 270 3.0TDI