is there a difference between the thickness of tyres put on 12's and 13's...... and if so does the 12' and 13's end up being the same size in total Diameter ??

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"Have you ever noticed? Anybody going slower than you is an idiot, and anyone going faster than you is a maniac."http://i129.photobucket.com/albums/p218/mini78s/theone.jpg

the profile will be different...lower for the 13s...(assuming thickness means profile)

but the outside diameter will be nearly the same if not the same...

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whether it's 12" or 13" the thickness depends on the width of the rim again whether it be 5 5.5 6 7" 's wide. with a higher width number you will need to cut and lip/flare the guards etc... plus you may need to adjust various things to get the correct geometry.

wooo hoooo im not the only person with stupid questions

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busier than a 0ne legged man in a butt kicking competion

Steady on - steady on
No such thing as stupid questions - all questions are valid although I have seen stupid answers at times although not necessarily off the Ausmini group.

Have a squiz at this - heaps of info and calculators as well.
http://www.carbibles.com/tyre_bible.html

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Cheers,
Rod.
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'63 Morris 850 (Old Secondhand CAR)