Hi everyone,

I have just swapped my old, worn out 275/40/20's for a set of 275/45/20 Grabber AT3's. (I was torn between getting the AT3's in the original 275/40/20 or to go for a set of 19" rims and fit 255/55/19 AT3's. Finances and the fact I didn't want to wait for the 19's to arrive helped me to make up my mind!!)

My question to you knowledgeable people is - what settings do I need to alter within the CCF to make it understand I have the new, slightly larger tyres?

I have been in and had a look and I can see I can change the circumference from the current 2220 to 2375 but what about the other settings -

CC KPH off-set? Currently set at +2.0kph

CC Speed Scaling Factor? Currently set at 3.5% (According to https://www.tyresizecalculator.com/tyre-wh...dimensions - the difference in tyre wall size means my speed will be 3.8% greater than the speedo reading do I change 3.5 to 3.8?).

Tyre (Spelt tire in the CCF ) Aspect Ratio? Currently set at 40%

Trip Fuel Eco Adjust? I have no idea what this means but it is currently set to 7.0%

Tyre Rolling Radius? Currently set to 359 - 275/40R20. There is no option that I can see which has 275/45R20. Do I simply select 375 from the list? That said, I did read a post about measuring from the floor to the centre of the wheel. Which is correct?

Tyre Size MSB - IPC? Currently set at 31 (Having also looked at https://tiresize.com/comparison/ it would appear this number may need to be set to 29.7 for the larger tyre?).

A lot of questions but I would really appreciate any advice you could give.

Thanks,

Andy.

Before altering the ccf, try checking your RRS indicated speed against your phone GPS. After fitting 275/45R20's my speedo is now as good as bang on where 275/40's were about 3mph below GPS speed. I'd expect a more accurate speedo would likely make the other settings more accurate aswell 13MY Range Rover Sport Autobiography SDV6 - mine
14MY Range Rover Evoque Dynamic SD4 Black Pack - wife's
99MY Defender 90 TD5, Soft Top Conversion - my toy, and bairns favourite

Don't bother changing anything it will not make any noticeable difference to any of it. The new size tyre will actualy make the speedo slightly more accurate.

Cheers
Col

Have a read

https://www.rrsport.co.uk/forum/topic57124.html 2006 2.7 TDV6 HSE in Cairns Blue.

Col wrote:

Don't bother changing anything it will not make any noticeable difference to any of it. The new size tyre will actually make the speedo slightly more accurate.

Cheers
Col