Hi all. Something has been bugging me for a little while and look to this forum to hopefully find an answer…

First of all, i am not running odd sized tyres nor do i intend to. This is simply a ‘why’ question and not a ‘can I’ question.

There was a comment made on another thread that has been niggling in my head for a while, where someone asked about swapping tyres, and there was the usual “don’t run different tyre sizes” responses.

The bit is rattling around in my head and I want to know is why is it bad? My experience of diffs is very limited and goes as far as Technic Lego, and having once driven a car with a welded diff. The diff, in its simplest terms, is designed to allow two wheels to rotate at different speeds from a single drive source and as such its why the wheels do not skip around corners.

So surely if their function is to allow the wheels to rotate at different speeds, then why are different wheel sizes a problem? The diff will allow this by their very design? And in addition to that, there is another diff between the front wheels and rear wheels doing the same job as the front and rear diffs.

Now, limited slip diffs have (i believe) clutch packs and I guess they could wear out with prolonged use… but normal (non-LSD) are just rotating ‘cogs’? The only thought of what the problem could be is the ‘cog’ that joins each wheel (or front and rear wheels for the central diff) bearings wear out?

Is it the limited life in that ‘cog’? Or am I missing something out… there must be something more to this!

Can anyone help to enlighten me? Currently driving: 2012 L322 SE Overfinch 4.4 tdv8

Past rides:
2014 Audi Q7 3.0d (good riddance)
2010 L322 Autobiography 5.0 Supercharged
2011 L320 HSE 3.0 sdv6
2014 Jaguar XF-RS 5.0 supercharged
2007 BMW 535D
2005 Mini Cooper S

https://www.fullfatrr.com/forum/topic59478.html

Similar to yourself with my understanding…. And history of transmission wind up on different systems of 4x4….

The only thing I’d add is…. Is it down to tyre circumference…. DSC…..

What about the traction control, DSC etc? 1 wheel could appear like it’s spinning if it rotates more than the other 3… and car could brake that corner maybe…

That said, there must be some tolerance for cornering, and difference in tyre size prob have to be big to go outside that tolerance….

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

Would you be covered by Insurence if you had an accident? probably not 2018 SE SDV8 4.4 Byron Blue

I get that bit…. Though a space saver is a different size and that’s ok insurance wise. Suppose it’s about what a car is type approved with, from an insurance perspective.

Physically I suppose you could fit anything you liked, but it’s the mixing of sizes and what it may do to the car and it’s systems I think we are meaning. I put LR 18s on to my Disco 2, but my spare was still a 16 inch. Had to use it once. Didn’t really notice any difference that I remember. 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

My understand as to why its bad is a diff expects the 2 wheels to be driven or rotate at the same speed for the majority of its life. The 2 wheels rotate at different speeds when you go around a corner.

https://cdn.thinglink.me/api/image/4781975...?wait=true

If you look the picture in the link above, when the car is going in a straight line, the cage rotates, but the planet wheels do not (the small green gears)

When you go around a corner the green wheels rotate in the cage. You will note that there is no bearing that the green wheels run in, there is normally a copper/brass or fibre shim behind them, that all.

If you had odd size wheels, these gears would constantly be turning, and wear the diff out much faster than usual.

To add to this all the electronics would probably get upset as the ABS sensors on each wheel will say that they are running at different speeds, there will be a tolerance built in to this, but who's to say what the electronics will do 2011 Range Rover Autobiography TDV8 Buckingham Blue

thanks for the responses so far

Insurance: yep, they’ll use any excuse to not pay out.
DSC: possibly, depends on how sensitive it can be. Having one wheel smaller than the rest may been seen by the computer as a traction problem and may try to compensate???
Wear and tear: this feels like a good reason to avoid mis sized wheels. Additional constant wear on an otherwise little used gears.

Any other thoughts? Currently driving: 2012 L322 SE Overfinch 4.4 tdv8

Past rides:
2014 Audi Q7 3.0d (good riddance)
2010 L322 Autobiography 5.0 Supercharged
2011 L320 HSE 3.0 sdv6
2014 Jaguar XF-RS 5.0 supercharged
2007 BMW 535D
2005 Mini Cooper S

https://www.fullfatrr.com/forum/topic59478.html

Will you - in theory at least - have a higher braking force through the smaller wheel, possibly making the vehicle pull to one side during heavy braking? The smaller wheel will be more liable to lock up? Assuming all wheels have the same brake components. No idea just how different the wheel sizes would have to be to make a noticeable difference.

While I don’t see a reduced braking force as the car itself is providing the weight and pushing down on the wheel, there could be a reduced surface area from the smaller wheel?
And, in theory, the smaller wheel could be lighter (in sprung weight) so may behave differently to the heavier wheel?

However all should be minor differences and as the braking system copes with different surfaces any, it should cope?

All based on my man logic, and actual technical knowledge at all… so buckets of salt ready for those pinches  Currently driving: 2012 L322 SE Overfinch 4.4 tdv8

Past rides:
2014 Audi Q7 3.0d (good riddance)
2010 L322 Autobiography 5.0 Supercharged
2011 L320 HSE 3.0 sdv6
2014 Jaguar XF-RS 5.0 supercharged
2007 BMW 535D
2005 Mini Cooper S

https://www.fullfatrr.com/forum/topic59478.html

If you run different sizes across an axle you will cause problems as highlighted by a recent thread in that if the ECU detects more than a 1kph difference on wheels it will interpret this as cornering and change gearbox behaviour I.e stop it changing up. Not sure different tyre sizes across an axle will pass an MOT either. If anyone has done the popular mod of fitting larger 255/55 R20 tyres from the L405 they will find themselves in this position if they have to use the spare which is a very good reason to replace it with a full size spare. As for diff wear I’d think the amount of time over the life of a car that it spends cornering or driving on non straight roads let alone the off road design would mean the diffs are pretty robust - but I wouldn’t want to test that assumption by running different size wheels. 2012 TDV8 Vogue (L322)

The gearbox thinking it’s cornering is a good one! Does this happen if just one of the two wheels show a slower speed?

And what about the variable dampers? Do they stiffen up thinking you are cornering? Currently driving: 2012 L322 SE Overfinch 4.4 tdv8

Past rides:
2014 Audi Q7 3.0d (good riddance)
2010 L322 Autobiography 5.0 Supercharged
2011 L320 HSE 3.0 sdv6
2014 Jaguar XF-RS 5.0 supercharged
2007 BMW 535D
2005 Mini Cooper S

https://www.fullfatrr.com/forum/topic59478.html

Variable dampers use accelerometers around the car for that, so they should not be affected, though I’m not sure if they do something under braking, but again, that’s prob the accelerometers again.

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

AFAIK the active damping is disabled under braking. 2012 TDV8 Vogue (L322)

A more basic and mechanical reason is this:

If you remember back to your LEGO Technic days then under normal straight line driving the centre gears of the differential do not rotate, only the outer ring gears rotate. When you corner the inner gears start rotating to allow the two axles to rotate at different speeds.

If you have different sized tyres then you are effectively always cornering and thus these inner gears are always rotating. This becomes a problem because the bearings and internal components are not usually specced to be 100% duty cycle. Equally the gears are not designed to take 100% of the torque to the wheels for a sustained period, only the difference between them. So technically it's not a limitation but more a practical one made through design decisions.

If you have been in a proper Defender or Stage 1 V8 then the transfer box should be locked sooner rather than later if you're going to lose traction because the centre diff can't sustain spinning for very long as it throws the oil out of the bearings. Ed

Cheers sako that makes sense.

So while we could drive with mismatched wheels, the extra continuous load on those inner gears is not recommended. So, swap to a spare to get out of trouble, but don’t drive too fast or too far.

Hence the speed and distance limits on space saver spares I guess??? Currently driving: 2012 L322 SE Overfinch 4.4 tdv8

Past rides:
2014 Audi Q7 3.0d (good riddance)
2010 L322 Autobiography 5.0 Supercharged
2011 L320 HSE 3.0 sdv6
2014 Jaguar XF-RS 5.0 supercharged
2007 BMW 535D
2005 Mini Cooper S

https://www.fullfatrr.com/forum/topic59478.html

The space saver should match the road wheels in rolling radius. 2012 TDV8 Vogue (L322)