Sorry folks but didn't know whether to put this in wheels or general section.

So I'm driving along the A303 going home and I'm in lane 2 and I look across at the Nissan Leaf with a digital speedo reading 70 (easy to read in the dark) that I'm overtaking, but I'm doing 70!. The car came with 20inch wheels with 255/55/20 tyres fitted, so I suspect these wheels have thrown the speedo out. How can I calculate how much it's increased and drive accordingly?.

RRS 2006 HSE 2.7

Thanks everybody Steve

Go here: https://tiresize.com/comparison/

Put the standard tyre (275/40/20) in the first size and the tyres you have as the second size. It will produce an error chart. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

How do you know that the Leaf’s speedo was correct? Remember that up to 10% error is permitted so the Leaf’s could have been over reading by 10% and yours under-reading.............. you would be more accurate to compare your speedo to the speed reading on a hand held GPS in the windscreen. 2018 Melting Silver Mini Countryman PHEV on order for April delivery, bye bye Land Rover after 21 years and 11 LR products..... all because you won’t give me reassurance that should my SDv6 crankshaft break because of a design fault, not MY fault, but the engine builders fault - it will be replaced for free.
2015MY Corris Grey SDv6 HSE Dynamic, but not for much longer........
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

I thought that too Tim. I've used GPS-indicated speed on my phone (using CoPilot software) and the Speedo is not far off. Interestingly, its accuracy compared to the GPS varies in a non-linear manner. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

OK thanks, I'll break out the TomTom and see what that says.

GPS does have problems with windy roads when calculating speeds, best to find somewhere with long straights where you can set the cruise and see what the difference is over several speeds held for a few minutes. I have done this in my Defender using both one of the routing apps on a iPhone and simultaneously using digital OS maps on an iPad in a cradle. They agreed within 0.1mph and my speedo on factory recommended MTR tyres was reading 4mph high at 50mph and 6mph high at 60mph. I am lucky to have a 6 mile long straight and flat A road between home and the local town. 2018 Melting Silver Mini Countryman PHEV on order for April delivery, bye bye Land Rover after 21 years and 11 LR products..... all because you won’t give me reassurance that should my SDv6 crankshaft break because of a design fault, not MY fault, but the engine builders fault - it will be replaced for free.
2015MY Corris Grey SDv6 HSE Dynamic, but not for much longer........
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

One easy way, if a bit expensive, is to drive in front of a speed camera at 100 mph.
You will receive a picture with all the informations you need

 '08 RRS TDV8
I converted my diesel RRS to run on an environmentally friendly mixture of caribou fat and baby seals oil

mime is the same showing faster at 50 60 70 had a look to reset the tyre size as i am running 275 45 20's but could not fathom out what size it should be as it said 353 but not sure if that is circumference/diameter or something else

Hi Steve,
the 255/55/20 are about 30mm larger on the radius than the 275/40/20

good for ground clearance
but that big a step up I would suspect your speedo now reads Under your true speed, so best get the checking against a GPS done sooner rather than later

let us know the result  Corris HSE Dynamic 2016

Always pick up stick in the middle !!

So just a quick question, the 255/55/20 tyres that are fitted (and came with the wheels/car on purchase http://www.hubcaphaven.com/p/21612/aly7225...44848.html), are they the correct size for the vehicle that they would have been fitted to "L494 HSE - RRS2" originally? or has somebody changed the tyre size for a reason......

On a previous post it was kindly pointed out that my car would have had 19s' from factory which one day I may go back to, but these look the part and not too blingy for my tastes.

Still learning about the car and what it should have and what not

Steve.

And just a quick look at the website that RRSTDV8 posted:

Speedometer Error
(Set 255/50/19 to 255/55/20 Size)

Speedo 20mph 30mph 40mph 50mph 60mph 70mph 80mph 90mph
Actual 21.4 32.1 42.8 53.4 64.1 74.8 85.5 96.2

Good website that .

Steve.

you can adjust the cars ccf to allow for bigger wheel circumference with iid bt tool

or a similar diagnostic tool with the same capabilities 

Hi,
yes 255/55/20 are a standard size for RRS2

along with 275/45/21 and for winter 235/65/19 both give a similar radius

I think standard on yours would have been 255/50/19 which match's to 275/40/20

I had 255/55/19 fitted (about half the increased radius of yours) and the speedo was spot on

 Corris HSE Dynamic 2016

Always pick up stick in the middle !!

The increase from 275 40 20 to 275 45 20 on my 2012 RRS (increase of 3.8% in rolling radius) made the speedo exact as well. 12MY RRS 3.0 SDV6 HSE Lux Sumatra / Black + contrast/ Grand Black
2010 Disco4 3.0 TDV6 HSE Nara Bronze - Missed very much
2008 Disco3 2.7 TDV6 XS Stornoway - Now gone