I like an experiment
Bloody scientist (me)  Gone to a good home: 2011 4.4 TDV8 Vogue SE Buckingham Blue with Ivory and clear glass = "Rory"
2025MY Defender D350 90 in Silicon Silver on coils
1974 Series 3 Lightweight = "Millie"
Many, many other Landies over the years
My preferred specialist: www.glenrands.co.uk
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Bloody engineer me, I need to know the reason why!!!  Got - 2017 SDV8 Autobiography in Loire Blue
Had- 2008 TDV8 Vogue SE in Java black
Had - 2007 S/C in Stornoway Grey

For interest I used this tyre calculator, https://tiresize.com/tyre-size-calculator/ and it seems that the profiles give the following revolutions per mile

255/55x20 - 650 revolutions per mile
255/50x20 - 672 revolutions per mile.

So if you had the 50's on the back they would be trying to overtake the front!! (or something like that!)

I would have thought that it might be giving the transmission all sorts of issues!!

In one revolution the 55 section tyre travels 97.5"
In one revolution the 50 section tyre travels 94.3", so the 55 section travels 3.2" further than the 50 section, that must cause some wind up on the axles.
If you put 22" rims on 55's on one side they travel 103.8" so you could do donuts!!!!

Or go around roundabouts really really quickly!!

alanm_3 wrote:

Bloody engineer me, I need to know the reason why!!!

There will be different tolerances for front wheel travel compared with the rear which I expect are biased to allow the front wheels to travel further than the rear which occurs when turning corners. Thus smaller front tyres travelling further are less likely to trigger a fault than larger front tyres travelling less than the rears.

The fault existing without travelling will be due to the need to demonstrate by a drive cycle that the previous fault condition no longer exists.

I'm astounded that anyone would mix tyres an inch difference in diameter on a 4WD, totally unlike different wear on notionally the same diameter tyre which is never an issue.

NB. Sensors do not self re-calibrate to accommodate unexpected results. ECUs are programmed to tolerate a range of values and when the value is out of range then a fault condition is logged.

Ok, so I reverse engineered my actions of yesterday (purely for experimental purposes obviously) by swapping the n/s front and n/s rear wheels. The result? No errors at all

Clearly the car doesn’t like what I did to it, so I’ll be investing in 2 new 255/55 tyres as soon as. Not that I’m doing much mileage these days - 50 miles last month But I’d hate to upset and confuse the old girl  Got - 2017 SDV8 Autobiography in Loire Blue
Had- 2008 TDV8 Vogue SE in Java black
Had - 2007 S/C in Stornoway Grey

Good to hear you're sorted. Maybe the old girl was just trying to tell you not to mix tyre sizes, as best she could, before it did more expensive damage  Current: FF Westminster money pit

Gone:
RRS 4.2 SC
RRS 3.6 TDV8
FL2 TD4

That's so unlike JLR to be so charitable  Got - 2017 SDV8 Autobiography in Loire Blue
Had- 2008 TDV8 Vogue SE in Java black
Had - 2007 S/C in Stornoway Grey

When I last had my tyres changed, the quite knowledgeable chap at Pro-tyre said that the sensors tend not to like a 2mm difference in tread-depth on a change for just this reason. Its possible that the system copes with gradual wear, and accounts for it. If you then swap your tyres, you've increased the differential between front and rear by a factor of 2x in one hit? Maybe that's why it throws a wobbly? FFRR 4.2SC/KTM 990SMT Current and cherished.
Honda Accord 2.4 Vtec touring. (Yeah, I know, think: Kids)
Mitsubishi Pajero Evolution Nut-wagon. and a Buell XB12R
TVR Griffith 5.0 (backed up with Vauxhall Carlton road barge for the weekly shop.)
Honda Integra Type R DC2 Possibly the best drivers car. Ever.
Mitshibishi FTO GPX MIVEC.
Honda CRX Vtec Mk2 -NO, Not the del sol!!!
Volkswagen Scirocco GT
Mini 1275GT
MG Midget (Heavily modified)
I'm going to stop now.....

31" mixed with 30" is a very good reason for abs to get upset whereas 2mm is just a tyre dealer trying to sell more tyres.

He wasn't trying to sell me more tyres, I didn't need them. He just put that into the conversation from his experience. FFRR 4.2SC/KTM 990SMT Current and cherished.
Honda Accord 2.4 Vtec touring. (Yeah, I know, think: Kids)
Mitsubishi Pajero Evolution Nut-wagon. and a Buell XB12R
TVR Griffith 5.0 (backed up with Vauxhall Carlton road barge for the weekly shop.)
Honda Integra Type R DC2 Possibly the best drivers car. Ever.
Mitshibishi FTO GPX MIVEC.
Honda CRX Vtec Mk2 -NO, Not the del sol!!!
Volkswagen Scirocco GT
Mini 1275GT
MG Midget (Heavily modified)
I'm going to stop now.....