I've been running 245/45 R 18 tyres on 18" Zenith wheels (ET50 or thereabouts) on my otherwise unmodified Octavia Scout. They don't rub, but come close when turned in about 15 degrees- leaving abot 5mm clearance between the front outside shoulder and the wheelarch.

I'm contemplating fitting some 175/80 R 16 Michelin XC4S taxi tyres on 6.5J ET45 wheels and have calculated that although a larger diameter, the front outside shoulder does not extend as far from the midpoint - so in theory there should't be an issue there.

EXCEPT the wheel doesn't turn about a point on its mounting surface or necessarily its theoretical midpoint.

Anyone able to help me improve my calculations perhaps with some advice about how much I need to allow laterally for the theoretical centre about which the wheel turns?

Thanks.

(The answer to "Why?" is "Because I hope I can!")

Please discount any speed or insurance constraints as my insurers are aware of my current setup and have confirmed in writing that they're also OK with Q rated taxi tyres providing they're not a performance modification! As that restricts the top speed to 99 mph and still exceeds the national limit they accepted that it wasn't enhancing performance to the detriment of safety.

Stop calculating and just trial fit them!

Stop calculating and just trial fit them!

Problem with that is I would have to buy them first at about £170 a corner. Thereafter the tyres are about £60 each, and I could substitute alloys for steel wheels, but the problem remains that to try I must first buy (and a single wheel won't be sufficient to test with).

Probably just need to know how many mm between hub face and whatever you call the point it pivots around.

Walter - If you're reading this I'd appreciate any input you may have

Your 18" Zeniths are 7.5J and have an ET of 51. If you go onto www.willtheyfit.com and put these details along with your tyre size, and then the details of your proposed new wheels and tyres you'll see you'll have plenty of clearance.

See told you, stop calculating and buy haha

Your 18" Zeniths are 7.5J and have an ET of 51. If you go onto www.willtheyfit.com and put these details along with your tyre size, and then the details of your proposed new wheels and tyres you'll see you'll have plenty of clearance.

Thanks Julie, that's a useful site.

Looks like the viewpoint is either straight on or from directly behind with steering at 0 degrees. The additional 4.35mm nominal ride height would be a close call if the tyre was the same width as the original, and when the steering is turned slightly I wouldn't want any less space than I cirrently have (this allows for a little shift in suspension when steering). The thinner tyre should mean this worsst case 4.35mm difference is actually less, but given there's currently about 5mm in it I'm trying to satisfy myself that the gap will be sufficient to prevent fouling.

The analogy is parking a longer but narrower car in a confined garage versus a slightly shorter but wider one. Parking straight on is no problem, but if you have to drive in from an angle, the front corner might come closer to the wall with one or the other - if that makes sense.

The difference between new tyres and tyres with 3mm tread is greater than the 4.35mm difference between your current wheels and the new combination you're looking at. I'm guessing that clearance isn't such a big concern that you'd only ever fit part worns, so I think you'll be just fine.

The difference between new tyres and tyres with 3mm tread is greater than the 4.35mm difference between your current wheels and the new combination you're looking at. I'm guessing that clearance isn't such a big concern that you'd only ever fit part worns, so I think you'll be just fine.

I can agree with that given that many tyres have 7 to 8mm of tread, but mine are still about 7mm and whilst good in snow they are no longer manufactured, and are about £180 if you can find them - making the thinner ones more cost effective even with a new set of alloys (before I even consider the proceeds from selling my current wheels if I wanted to).

Its when the tyre/wheel is turned slightly that the outside front shoulder comes closest to fouling the wheelarch, hence trying to work out the diagonal distance to that shoulder from the centre of the tyre - but that's not quite the point about which the wheel turns for steering.

Going to start confusing myself here if I'm not careful.

I think you're really overcomplicating this. If you are really concerned by this extra 4.35mm then turn your wheels to wherever clearance is at its tightest, and then slide a few pages from a magazine (4.35mm worth) between the central 175mm of tyre and the arch liner - that should prove it one way or the other.

I think you're really overcomplicating this. If you are really concerned by this extra 4.35mm then turn your wheels to wherever clearance is at its tightest, and then slide a few pages from a magazine (4.35mm worth) between the central 175mm of tyre and the arch liner - that should prove it one way or the other.

_{That's effectively what I did earlier when i stuck on a Post-It note that I had cut to stand up 5mm and marked 35mm in from the existing shoulder - i.e. where the new shoulder would be.}

_{My son has just done Pythagoras at school, so to counter the eternal "Why?" question, I thought I might show him that there's a practical application for a lot of what he thinks is bo11ocks. I'd look daft if I got it wrong.}

_{Thanks to everyone who's let me bounce ideas off them all..}

_{My son has just done Pythagoras at school, so to counter the eternal "Why?" question, I thought I might show him that there's a practical application for a lot of what he thinks is bo11ocks. I'd look daft if I got it wrong.}

Give him a length of cord of length 12 and tell him you'll swap it for a £20 note if he can use it and a pencil to draw a right-angle.

Give him a length of cord of length 12 and tell him you'll swap it for a £20 note if he can use it and a pencil to draw a right-angle.

Just shows we're thinkers in Scotland. Maybe it just rains too much to encourage the easier physical check.