Michelin TYRES are much cheaper in Thailand. Consequently, I intend to drive to Hatyai and change my existing worn out tyres to a set of new michelin tyres. I have been informed by a retired custom officer that so long as the tyres are affixed to my car albeit they may be new tyres, there will be no tax payable. However, he claims that sometimes, the custom officers at the border may insist that the TYRES are dutiable and will insist that tax be paid. When this happens, I should politely remind them of the correct position. Does anyone know who is correct?

it doesn't make sense if they ask you to pay.
you're using it, not bringing back for business

The issue is whether such goods already affixed to a vehicle are still subject to payment of excise duties as you rightly pointed out, the items are not for resale but for my personal use. If anyone can help to quote relevant sections of the law to avoid having to involve ourselves with an argument with the custom officers. I also intend to have my truck fitted up with bedliner, cabin, anti-roll bars, shock absorbers and coil springs etc ...... I.e. having it fitted to prepare for an over land trip abroad.

firstly, what proof do they have to accuse you of changing new tires?
if you can't answer this question, then case close

Unfortunately Michelin tyres made in Thailand are clearly embossed on the tyres as being manufactured in Thailand. You are right that the customs cannot differentiate between tyres purchased from Malaysia or directly from Thailand. A friend of mine was asked to show receipts from Malaysia and when he could not do so, he was asked to pay an amount which would make it not worth the while for him to buy from Thailand. Not wanting to argue with the customs who gave a veil threat that he could detain his car, my friend had the matter settled amicably on the side, which was still a win win situation.

I am just interested to know the correct legal position to avoid having to undergo the stress of having to argue with our customs.

This actually depends on your luck, which officer is on duty, etc.
They just want to get some money out of it.
If they verbally said they can detain your car, ask them for the evidence on papers.
Under which article its not allowed.
If i were you, i will fight for my rights